You could easily get 2 5G rectangle tanks 11.99-14.99 I have seen at a big box lfs, and from a lfs or online, 2 Sponge filters - one large air pump with a double gang valve.
My experience with a divider was that the sneaky Betta jumped over one time and then the littler one next door somehow found a way to slip under the divider in a tiny space where the sand had moved back a little bit. Found cowering in a crevise to stay away from the other bigger guy. For me not ideal. It was an Aqueon mini bowfront with a divider. Maybe the same as you are thinking of.
I wouldn't recommend it, not really enough room. I found that with a half moon Betta the 2.5G wasn't big enough for one Betta even.
Bettas love swimming around. Not really much room in a 2.5G.