Several of my males are kept in split 10 gallon tanks, so they see each other all the time. I don't really consider it an issue because they've all gotten used to it and rarely have an issue with the other betta now.
When I stuck Riot (black HM) and Xiphoid (red VT with long pec fins) next to each other, there was a bunch of dancing, flaring and tearing back and forth next to each other at the divider for about a week, and then they both apparently figured out they weren't going to get each other, and now spend most of the day swimming around their section of the tank happily, and flare at the other betta maybe three times a day (primarily feeding time if one of the pellets gets too close to the divider before they get it).
In the past I had a pair of bettas in the same situation (divided 10 gallon) that flared at each other for two days, decided that was getting them nowhere, and resigned themselves to blowing their bubblenests against the divider for the rest of their days in some strange competition about who could make the bigger bubblenest. Every pair reacts slightly differently to their new neighbor, but I've yet to have any males that have ever been so aggressive that they've hurt themselves trying to get at another fish (now getting at the shiny thermometer is another story entirely).