It's best to mention it to both employers, but be careful and check your contracts. When I worked at WHSmith there was a clause that said that I had to ask for permission from a manager before I could get another job, and if the permission was denied or I didn't ask for permission (and they found out) and I still took the other job, I was liable to be fired. To be honest, though, I doubt there will be conflicts of interest if you're temping in a hotel and working part-time at Domino's, so they shouldn't have a problem with it (but do tell them).