Advice needed HELP!

I have 2 male cats one is a 7 month old white cat (we got him first) the other is a 5 month old Tabby (we got him second a few months later) when we got the second one our white cat tried to i guess mount him which we thought was strange but after reading I guess it’s a way to establish dominance?? Anyway they eventually got along & they still get along very well. About a month ago or so I noticed my white cat mounting the tabby cat again like he actually gets on top of him but I don’t see any penetration (thankfully) anyway what I’m trying to figure out is my white cat in heat (they’re both not fixed) or is it still about dominance??