My two cents about it...
In my opinion, the main things two practitioners of Mahayana and Theravada must accept before any conversation are:
Each one considers his tradition best for himself and perhaps (not always) others. Otherwise why would they practice it?
The Theravadins consider Mahayana later developments of the teachings, never revealed by the Buddha, not always accurate and by any means consider Mahayana superior. By the opposite, they consider Theravada more correct.
The Mahayanists consider Theravada teachings incomplete, although extremely wholesome, but philosophically less accurate.
If both accept this and that both are in different circumstances due to the effects of past actions, who is right or wrong matters little when it comes to respect.
There will be differences and incompatibilities. Trying to make all traditions equal is the same as respecting them just because of that, not in spite of their differences. That's just a subtle form of intolerance.
If people ease up, accept the differences and move on, recognizing that there is great virtue in practicing Buddhadharma, whatever it's form, then I believe friendship can consolidate. The subtle doctrinal differences matter mostly to very advanced practitioners. Whatever the result of the practice of these two traditions, it is always excellent compared to our current state... so one will win a Ferrari while another will win a Mercedes. Who knows who will win the Ferrari? Right now we only ride on a broken skateboard.
I believe that practitioners of different, yet outstanding traditions, as Theravada and Mahayana not being able to harmoniously communicate only shows great immaturity. One can criticize the view of each other with fair play, but should be careful when speaking about the path or the practitioner.