i have 2 bettas in a 5g but one never had a full 5g tank to themselves. i had the divider setup before the first one went in and he never explored the other side. i dont know what divided a tank leaving the first betta with 50% less territory would do but they may get used to it. probably would need to ask a more experienced betta owner here for that.
i wish you the best of luck in your plans to move the betta to safer territory.
Kigurumi (F) yellow PK 7/4/13, Kurama VT 9/8/13, Zeydalus Salamander EE PK 21/3/14, Ryuk turquoise dragon DeT 8/5/14----Crested Gecko (8ths old) Sven Cal 27/12/11,
RIP: Osiris copper DT, Nel blue VT, Nelliel turquoise grizzle DT, Zaku red cambo, Blitzkreig Pineapple CT, Halloween orange chocolate DeT, Grimmjow, turquoise green CT