Hello Self_Optimization,

 I'm (NeTi) and my partner is (NiFe), and we have worked on developing different functions for a couple of years. So, I thought I could answer this question to a certain degree at least.
And this answer is based on our personal experiences, if you will be wondering.

As you mentioned, functions have different information representation and processing methods, and moreover they also have different information selection.  Therefore, my best tip wouldn't be to try to use them simultaneously, but to grow your familiarity with them individually. At least for a good while.

The reason being that even though all functions are active and available, even when you aren't conscious of them, they might not have that much influence or say in matters. And for functions to be readily accessible, they need to be well-developed. 
Fe is my third function, and in the last two years, especially, I have done a lot of things to further strengthen and develop this function. It is still a willful effort to use it directly, to be in Fe. 

To answer the question in another way, it has not been possible for me to be in two different functions at the same time, at least consciously. Maybe it is an individual limitation, but I have no tip for achieving that at least. But even though I might be able to be in only one consciously, I am also consciously aware of the rest being there when I glance at my subconscious.


Kindly,
Caerulea-Lawrence