Can't you book the ticket on expedia/orbitz? Then you have it all on one PNR.

Either way, bags must be rechecked at the US port of entry.

You can also ask at MAN (armed with both tickets) if they'll tag bags all the way through. It seems your best bet is to have EWR as your entry point (longer layover) so you can recheck all the way through to GCM, if the bags are already tagged. Or they may tag though to GCM if MAN refused.

If not, it will be annoying to leave the secure area in MIA, get bags, recheck, and reclear security.