Yes, but isn't it easier just to put something between two tanks if they are next to one another then having to worry about stressing them?Blough said:
With two ten gallon aquariums you still want to make sure that they can't see one another, if the tanks are next to each other. Seeing another betta all day every day can cause a betta to be stressed out. Just because a betta isn't flaring doesn't mean he's comfortable, it may mean he's tired.
While seeing other bettas can cause stress, it does not do so for every individual. As long as each betta has a "safe" place (a cave of some sort facing away from the other bettas) in their own section that they can retreat too and feel comfortable in, it normally is not that big of a deal that they can see each other through a divider or tank glass. Bettas are pretty easy to see stress. They will hide all the time, bite their fins, clamp their fins, or become really lethargic (without any other explanation).
That all depends on how things are for the OP as to if it is easier. Sliding a piece of paper between the tanks is an easy fix if one is really concerned. However if the tanks are set up well with a place for each betta to get away, then it really shouldn't be an issue.