Whoops! Sorry my wording was a bit off...
They are both in different 2.5 gallon tanks.
I have an extra 10 gallon tank that I use to measure the medication (I use what I need and keep the rest for the next water change). I loosely cover this tank with Saran wrap so that there is still a bit of air flow, but not too much, and to prevent dust particles and dirt from entering the tank. When water evaporates, I simply add more (the meds don't evaporate, I hope).
The yellow one's name is Anna, named after the seller who sold me the bettas for lack of a more creative name. The smaller one (melano/blue) was named first actually, by my baby sister who called it "Shrimpette". Shrimp is a synonym for small (in some contexts), and because she is female, my sister felt the need to add an ~ette suffix to her name, hence Shrimpette.