Pressing in bearings would seem simple enough. Put your press through one cup, then your frame, then the other cup, then tighten it, and ouala! Your cups are installed. But pressing both sides in at the same time means neither side of the press is sitting against a fixed surface, so it's not perfectly perpendicular to the direction the cups are sliding. And if there's anything we know about press-fit bottom brackets, it's that they kinda need to be perfect or you can get creaking and damage.
The trick is to be patient. Load up just one cup so the opposite side of the press can sit square against the frame. Then, once the first cup is installed, it'll be square enough to properly center your tool and you can press in the other side nice and straight.
Use this technique for bottom brackets, headsets, and any pair of matching bearings.