Yes, bettas can, do, and will jump so its a good idea to have some kind of lid. I would go ahead and put them in together divided if the bowl is unheated (the lamp doesn't count.....lamps heat unevenly and unreliably, they can easily over-heat a bowl. And when you need it the most at night you can't leave it on anyway)anyway though. Its fine for now to do that so long as you keep up with your water changes(I would recommend 2 50-60%s and 1 100% at least per week), you'll just want to upgrade/get a separate setup within the next couple of months at the most. :)