Transparency is a feature that some better image editing programs have that would let you see through one image in front and and work on the second in back(called layers). I know both GIMP and Photoshop have this feature (not sure about other programs). Basically you'd put the first mask on the bottom layer to be worked on, Then open the second mask as the upper layer and make it mostly see through but still visible. Then turn the area under the second masks textures black on the first mask.
Note: before saving the image get rid of the top layer (the see through one).