Cooking two pieces of meat in pressure cooker I know that I need to cook beef for 15 minutes per pound (500 g) in the pressure cooker for well done.

So one 4 pound (2 Kg) roast would cook for one hour. But what if I cook two 2 pound (1 Kg) pieces in the pressure cooker at the same time?

Would it still be one hour because it's a total of 4 pounds or would it be 30 minutes because each piece is only 2 pounds each?
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.)