With one cab and one mic? Never. You may get some comb filtering from the refelctions of the room, but thats not *quite* the same (though based on the same principle).
2 mics and one cab, all the time, to some extent. It cant be avoided. Some phase discrepancy will exist.
But its not always a bad thing. A huge part of metallicas guitar sound, for example, has always been phase cancelation of certain unwanted frequencies with different mics certain (specifically chosen) distances apart. Its a usefull tonal tool if you know how to handle it.
