I don't understand this bit can you explain in more detail please? Why is it so dangerous to do those 2 together but not separately?
Sure
Firstly, the DB bench press is an excellent exercise in it's own right, as with all exercises there are pros and cons, and as with most things, we want to do "everything" when just doing a little bit is what's called for.
we get too hung up on being perfect, or making sure your training is symetrical and in-balance or that it looks good on paper.
Young blokes screw it up by adding more to a routine when things are going well, don't.
The barbell press is an exercise that allows one to move a lot of weight, it doesn't need assistance, once a week is enough, what the bench press does need is a healthy shoulder, we all know what makes an unhealthy shoulder, if you don't you're in the wrong game.
I would say that the DB press is much more important to the lifting hobbyist that the barbell press, along with a well thought out routine for the individual, rather than some cherry-picked one.