That New (and cheap) 2.5g tank looks like what I need...a couple of those NPT style...dirt, sand and lots of plants ...couple of caves/castles/china tea cups...yep, I could definitely see that. I just got a bag of aquatic plants; I won an assorted package on aquabid and I am very impressed...$14, I believe and it's gallon ziplock full and bulging...nice and clean...12 different plants but multiple cuttings of each!
Sorry...was just delivered while I was typing the beginning of this message...got a bit off track :)
Dividers: if both sides are heaviily planted, even if one jumps to other side, the other betta usually can hide long enough.
In addition, if you make divider into a moss wall or plant a stand of something tall like vallisneria along the divider or maybe something bush such as hornwort or cabomba, the bettas won't get stressed by always seeing each other AND they will be less likely to jump it.
Not enough decorations/hiding places or too much space: your betta will appreciate having more places to hide or get away from perceived constant attention. Bettas like to pay attention to world outside their tank but they also like quiet time where no one can see them. They might be attentive to you and "talk" or "beg" but they probably still want a bidet hole to rest in. Heavily planted areas in your tank will do this. IMHO, live aquatic plants are the best but silk plants and additional houses work too!