They may or may not hang out together. Though, Boraras aren't much of a schooling species unless they're in gigantic tanks and have a reason to create safety in numbers. Otherwise, they spend 99% of their time apart, exploring.
Mixing two types in a tank this small will look poorly planned and less natural.
Some fish seem to do better in odd numbers when in smaller groups (though, some people say otherwise or have never observed it) of fish - like 21 Boraras brigittae instead of 900 Boraras brigittae. In the wild, fish aren't always going to appear in even, set groups. So having an odd number when you have a decent sized group can appear less planned.
It's also about aesthetic in smaller tanks and trying to keep things looking natural or beautiful and unplanned. It's why people use the golden ratio instead of centering hardscape in a tank.
Originally Posted by AdamC13
Are you saying the two different types would school together making it look mixed up? Also any reason you seem to be recommending odd numbers or is that just your preference? My OCD hates odd numbers when it comes to fish hahaha.