Cooking two pieces of meat in pressure cooker
There is a complicated formula for calculating cooking time based on energy input, surface area, thermal conductivity of food etc. Smaller pieces with a larger surface area will cook slightly quicker than large single pieces

In general for roasts (large slabs) just add the weight of the pieces and cook for the minimum time recommend by your pressure cooker for that (or from your experience). Then test using an accurate thermometer and cook some more as required. In my experience most people overcook meat anyway, so try it a little more undercooked for a change. Remember to let it stand at least 5 minutes per 500 g too

There are too many variables to give a blanket answer (stove energy, pressure in pressure cooker, meat type, fat content etc.)