Not sure where you're traveling but there's quite a few countries where it's standard, often required by law, that you provide identification for the occupants of your hotel room. In some cases they may specifically require a passport copy for non-residents. I don't think this would work in that situation. Unless, I suppose, if it was just applied digitally as a credit and they didn't amend your stay based on the guests at check-in.

Also some countries or properties have higher rates/service charges/taxes for double occupancy, something to look out for.