Alright I see Zhuge Liang was able to do that and so were a few others but it seems it may have happened, in my opinion, due to some sort of error in the ritual that may have prevented him from becoming a corporeal person in the world again. I'd prefer you have two separate entities as otherwise it'd put you into the direct war path of the other servants and it would make it harder for some to identify the true hero that resides within the master.