Lucky, you wouldn't have to keep them seperated for any length of time but enough to break up their routine.
As for the level of training and the trainer being a man, it's possible. Respect is key with horses, specifically like yours and mine. They sound very similar and it has taken me a long time to get to this point with my mare. I have a gelding that rides fine with women but not a man, he's got to prove himself first!
As for English and Western, there's not that much difference but if the horse was trained by a 12 time world champion, he's taught him certain queues and the slightest leg pressure in the wrong place makes all the difference. The biggest difference I see between English and Western is the riders legs. English is more straight and Western is bent like sitting in a chair (extreme example but hope you understand).
It may take some time but I'm sure y'all will figure each other out.
"Dream big, Work hard and Have FAITH"
"When my story's told, I hope he'll say well done and welcome home." ~ Templeton Thompson