sometimes you could use two protectors/packers on 1 program, but its just stupid and really doesnt add any security.

yes you can modify a packer to make it maybe harder to detect in PEiD/Foil Some Generic Unpackers, a generic oep cryptor was posted a while back that would do something like that, although it was quite unstable. It really wouldnt make programs any harder to unpack tho..

In regards to Xprotector, keep in mind you'll be rebooting your customers PC and the crackers PC so in the end your no farther ahead cause no one will want to buy your software. Altho to be fair to Xprotector, its optional if it reboots your PC or not but in any event programs run so slowly protected with it its useless.