Merging two water supply lines
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.