Create new map of Rx A, add to current map Rx B so they're both on the main map. Right click on the main map and Edit Layer so you're editing Rx B since it's the top layer. Go to the right side and use Copy from Layer. It should already have Rx A chosen, if not pick that one, hit select all at the bottom and keep hitting next at the bottom until you're at the last page and then choose Copy Layer and Attribute Data the second option down and click Finish. 

That puts both Rx on the same "layer". You can click save or file - save as and you will have a new file that's a larger combined one.