I do something similar under these circumstances, but I actually do use the IPTC Stationery Pad, but with any fields that aren't shared between the two shoots unchecked. So, PM still applies my copyright and contact info, as much location info as can be shared between the two shoots (ie, same state, country), and any keywords they might share. I then go through and apply keywords separately, and also caption separately. Not sure which works out to more work, but the system has worked for me so far.