I think it is fine to train and inject that muscle group on the same day, the only thing I wonder about is if it makes you suceptable ( spelled that wrong!) to muscle tears which I totally fear. It defintly adds size though for a few days to that group.
There is one Pro interview which I read and this guy took alot of shit, this is what he took everyday mon through fri and took sat sun off
500mg Sustenon
400mg Cypionate
100 Suspension
50 mg dbol
150 mg anadrol
and he said whatever bodypart he is training that day is where he inject everything.....This guy is a freak, it might have been on here that I read that.