It sounds like your brain is enduring what's called the source-monitoring error. This is quite common among most people; stress, anxiety, information overload among other causes. 

I'd suggest two (2) things. First, put a good amount of cards (at least six (6) - seven (7)) between these two (2) birthdays (or whatever they're standing in for as examples). This reduces the chances of your brain grouping them together, and then encoding the memory incorrectly. Second, there's been some work done on LessWrong regarding reconsolidation, you might be able to find some interesting help there in 'resetting' your mind when it comes to their two (2) birthdays. 

If it really is as simple as birthdays though, I'd suggest just practicing the cards at different intervals instead of at the same time a week later. Your brain is associating the time of revision with these numbers, instead of associating July 5 with Friend B and June 5 with Friend A because you've told it this: 'Agh, it's these damn similar birthdays again, I have to remember it this time!'

If it's more complicated than that, and simple isn't better, use the first two (2) methods I outlined. 

Hope this helps.

Medical student here, I get that a lot it's called interference at least in the supermemo sphere.

My personnal solution to this is to add more cards. For example "is friendA born before or after friendB?", "what are the birthday of friendA and friendB?".

The latter question is a crucial example actually. It makes you practice the recall of the distinction between the interference instead of the raw recall of each datum.

Also, as other suggested here, mnemonics help a ton: for example is there an intuitive reason you can link to friendB having an odd birthday and friendA having an even birthday? If so, add a third anki card to never forget the mnemonics.

You also might be interested in the 20 rules of formulating knowledge by supermemo.

One insight I had over the years on anki is that because of the algorithm, having more cards is not penalizing.

Also I created AnnA - Anki Neuronal Appendix to help spread reviews with semantic similarity if that helps.

Try memorizing their birthdates (including year).

That might be different enough from what you've previously tried to memorize (month & day) to not get caught in the tangle that has developed.

If you can remember which friend is older, or whose name comes first alphabetically, or something, you can associate that with which has the earlier birthday.