The first organizer enables a meetup to happen at all. The second organizer is a large boost in what the meetup can do. They allow the organizing team to be in two places at once, paying attention to two things at once. They let both parts take weeks off.
There is a drop in marginal utility, but it's not linear. In some cases I think the second organizer is pretty close to the usefulness of the first. If you would be tempted to organize a meetup if nobody else was doing it, but there's someone doing it so you don't organize, I would like to encourage you to reach out and offer to help on a regular basis anyway.
You have now read the basic point of this post. If you want to read on, cool, lets talk about organizers for more words than are strictly necessary.
I'm going to keep saying 'second organizer' and assuming you are the first organizer, and you have exactly one extra instead of two or three. Adjust as needed.
If you have a regular meetup cadence such as once a week on tuesdays or every second saturday of the month, a second organizer lets you miss meetups. This is invaluable if you travel, or have a busy work week, or have relatives in town. The first use of a second local organizer is they can run some of the meetups and keep the streak going. They can (and should!) do this even when you are in town, both so they're in practice and so you can relax and enjoy a meetup mostly like an attendee sometimes.
During a meetup you're both on the ball for, the second person means two things can be done at once. Something I commonly run into when running a regular local meetup is wanting to let people in the door, welcome them, and get them oriented to what's going on, while also trying to keep some kind of workshop or activity on the rails. It's just really hard, bordering on impossible, to give a relaxed and friendly "hey, good to see you, glad you made it! Do you mind taking your shoes off, there's a rack near the door right there-" also watch a timer for lightning talks and stop speakers at the right time.
For larger events, I've found it works best to break down the event into major parts. A Secular Solstice for instance involves a lot of creative decisions (making a program, rehearsing with singers, etc) and also logistics decisions (find a venue, announce the solstice in the right places, etc) so it can work to say 'you do the creative side, I'll do the logistics.' Make a todo list and mark your names next to each task. It's not that you can't or shouldn't do tasks not on your side of some organizational line, but it's easier if you know the kinds of ball you should have your eye on.
Different organizers might be better at different skills. Some are musical, some are good public speakers, some have a knack for finding great essays for the reading group, some are amazing cooks. More organizers mean you can each lean more into the parts you find fun. Ideally you at least know how you would do the steps of the other roles, if only in a basic way.
Perhaps most importantly, a second organizer enables a smooth transfer if the first organizer moves to the bay gets hit by a truck or otherwise stops running things. It's just way easier for a group to continue if a second person has access to post on the mailing list, knows what to do in order to schedule a meetup at the usual venue, and has run a couple of the popular kinds of meetups themselves so they aren't lost for ideas.
Ask.
If you've been running for at least half a dozen meetups, probably you have a sense of the most engaged couple of people. As you're cleaning up, or as you're waiting for most of the group to arrive, ask if they'd be willing to help organize. Be prepared to give a quick summary of what it involves. I find anecdotes of how I got started organizing help.
If someone says yes, then onboard them on the tools they might use. This is probably just making sure that they can post to the mailing list, but you might also show them or hand them a copy of any (or split the stash of) materials that are useful. Examples: adhesive name tags, portable easels, popular icebreaker games. Also consider introducing them to the hosts of your usual venues, or walking them through the process of booking the space.
One note: I've found that for an individual event, it helps a lot to have a lead organizer. Triumvariates of three equal partners, or councils of five, or other such arrangements seem to founder more on discussion and decision paralysis than they gain. You really don't want to replace a bus factor of one (yourself) with a bus factor of 'any one of these five.' That's worse. I suggest a structure of one lead and any number of secondaries. In either case, it should ideally be obvious who those people are.
For ongoing groups, my observations suggest the other way. It works fine to have an informal most active organizer, who makes most of the decisions, but if you can it's healthy to work in a secondary person who people know and who could easily announce things on a second channel; one example includes having a mailing list and a discord with different admins. What shouldn't happen is the second person silently copying a mailing list and lying about that, but a friendly or at least neutral parting of the ways (with probably lots of overlap in attendance!) seems good.
If you are an attendee, and there's just one organizer, and you'd be happy to organize if there was no organizer, please tell the organizer that and offer to help.
If you are an organizer, and you're working alone, and your attendees seem to be enjoying your meetups, ask one or two of them if they'd be up for running some things. Try to be specific in your request ("I need someone else to run the weekly meetup for the first two weeks of next month" or "I need someone to handle letting folks in the door and setting out the food for the weekly meetup next week while I run the workshop") and also to leave open the door for things they might be game to help with. ("You can run pretty much any topic you want- you like talking about entomology, want to give a lecture on that?")
If an attendee says "I wish we ran meetups like X" that is a good time to inform them that somebody can - if that somebody is them!
Oh, and if you're looking to find just one extra organizer, I suggest running Oops Its Time To Overthrow The Organizer Day.