Males should not be allowed to constantly see other betta in close range (tank next to each other). Sooner or later they will either loose interest and become.... lazy(?) or become stressed. IMO bettas that doesn't move much is vulnerable to diseases. AND it will be reluctant to spawn.
Males kept in solitary have more self confidence and will always be interested in other bettas, thus easier to spawn.
Females almost always carry eggs. Even small/young females may carry eggs, though they're not ready to spawn. When these females are kept in solitary, they often show breeding stripes. But when placed together with a male, they run. The key is to understand when an individual female is ready to spawn. Common signs are often discussed so I wont get into that. It makes them more interesting - sometimes very thin looking females would almost immediately spawn while the fat/full of eggs wouldn't.