"Roger Mills" wrote
Have to disagree here! My father-in-law did just this (added a second ball valve on a 15mm tank feed). His shower was draining the tank faster than the ball valve could fill it, so he fitted a second one. A lot of these ball valves have a reduced internal nozzle which will reduce flow significantly even on a 15mm feed.
Try it - it won't cost big bucks, but make sure you measure the fill rate (by rough check of the tank level change) with one valve open, then with 2 valves open.
HTH
Phil