Yes they can be seperated. They used to interact a lot more than they do nowadays, since Tiga has started to mature a lot they seem to have been doing their own thing!
I'll be very surprised if they can be kept together as full adults- it just doesn't normally work with Orangutan males and I suspect they will have to be split eventually.
Rajang's own father was either Jimmy or Paul, one of two Sumatran males who did live together at Chester as adults, but eventually they had to be seperated due to fighting and I believe in fact Jimmy later died from an infected bite he received during the fight.
I've always thought Rajang's father was Paul- they look very alike.