I think they can sense each other's hormones in the water, so I guess he realises there's something there.. Or he's just really curious. :/ Maybe you could try putting a mesh down the middle covering both sides halfway from the top like a roof very snug on the divider, so one can't jump and get to the other..
"You are forever responsible for what you have tamed."
Betta love: it's an international thing.
Azure, Sterling, you will be with me forever. Peaches, my crazy girl, I'll miss you the most.
The Boys: Tranny, Inky "Yume" Zuko, Mao, Onishiki, Chopin, Maelstrom, Kabuki
The Girls: Kamikaze, Beeuty Qveen, Beefcake!, Chunkers, Awesome Sauce
Meeces: Koko, Bijli, Baadal, Barkha
Turtle: Hi Handsome (female -__-)