the biggest guy in my gym does both on the same day and he's a monster, lifted 220kg for a single. so it cant be that bad for strength or mass, small excercise choices arnt going to make a huge amount of differnce to ur physqiue so long as ur working each area with solid compounds, concentrate on getting ur food/protein intake upto scratch, and finding out what volume works best for u will help add the most mass to ur physique.
When I was stuck for awhile on 185 for my bench max and doing the same reps for my high rep sets I started to do DB Flat on the same day as my BB Flat. My DB Flat went up from 55 x 8 to 70 x 6 in less then a month. My BB Flat Max went from 185 to 200 (yes put the 2.5's on to get to the big 200).
I saw great gains doing them in the same day. Sadly my gym for this 2nd semester doesnt have dumbells heavier then 45 pounds so I dont do them as much. I still do high rep incline with the dumbells here but definitely miss my flat dumbell bench press..