I am far from an expert but will give you my understanding...

All that will happen really, if you do them both same day, is you will not be able to work out the smaller muscle groups quite as well as if you had done them with an opposite group instead...

Example:
If you train chest and Triceps the same day, you may find that your triceps (Done after chest ideally, never before) wont be able to lift as muchlater when you train them individually than if you did them on a Back training day where they did less work.

However, some believe that this wont have a major impact on the development as long as you still do them to failure and get a good burn going as all you have actually done whilst chest training is pre-exhausted them.

Hope this makes sense and helps.