I am planning the remodel of a master bathroom that currently has a bathtub and separate but small shower stall. I plan to enlarge the shower and do away with the bathtub altogether, leaving me with an extra set of 1/2" supply lines. Should I just cap the extra set of lines, or would merging the two sets of 1/2" lines into a length of 3/4" or 1" pipe provide any advantage in terms of increased water flow/pressure to the new shower? Assuming that the inputs on most shower valves are 1/2", I would probably have to use a reducer to mate the relatively short length of larger supply line back up with the valve.
If you are going forward with the remodel to the extent that you describe the walls will probably be open so you can see where the existing water lines are routed. Due the proximity the existing tub and small shower it is likely that they are plumbed from a common feed line anyway.
I would recommend that you remove the no longer needed branches in the line.
You did not specify but there is a possibility that you may need to reroute the water lines to accommodate the new shower location anyway so this is an opportunity to make a clean installation that minimizes the amount of piping runs.
Probably better just to cap the ones you do not need. Or if you can without totally demolishing your floors and walls.. trace it back and redo from wherever it currently splits.
Trying to combine them probably wont buy you much pressure wise especially if you have to add extra bends or T-Pieces to fudge it together.
Further, you could end up with a system that oscillates quite nicely at certain flow rates because of differential pressure from each pipe. It could make quite a noise behind the wall.
Further, for the hot pipe in particular, you would be creating a loop which, as it heats up, will need to expand. That expansion could break a join.
Keep it simple...