I have two Kings in a divided, planted 10 gallon and they seem to be very happy and active all the time. I also have a King in a 5 gallon because he was alot more aggressive than my other two. I thought he needed his own place but now I think he seems to be a bit more bored than the other two. The other two like to look at each other through the screen (They don't flare, they just go to see what the other one is doing on the other side.) All my bettas have a nerite buddy too. Roku is VERY attached to his snail... he tail bites if I take it out.
So! I think that 5 gallons is okay, but I would like to give them 10 gallons. Two of them seem very happy in their 5 gallons. I have to play with Seraph A LOT to keep him happy. We tried the ping pong and bottle cap idea. He just LOVES people attention. He is also my betta that will flare if he isn't fed right when I wake up (he's in my room so he sees me get up) Anyway, I don't think that his boredom has anything to do with tank size. He just really loves attention :)