The key is how much will be running at the same time. One 1/2" pex line feeding two showerheads is close to that tubing's max flow depending on the hot/cold mix. 3/4" pex is somewhat larger than 1/2" copper, and 3/4" copper is probably a good size for a bathroom group. If the pressure drops on the cold side (sink and someone flushes the toilet), that may be enough with 1/2" line to cause a pressure balance valve to throttle the hot side while it's also feeding the shower. Pex is good because it uses fewer fittings, and thus has less friction loss, but volume still depends a significant amount on ID, and copper beats it size-for-size.