I had to do it at the last gig. My flats were finally dying, especially the E string. I already had a Super Steels 138 on as a B string, so I got the old, dead steel 105 I had taken off not too long before and put it back on to finish the gig. The rest of the strings were Fender Flats 80, 60, 45. Because all the strings were steel and not chrome or nickel plated steel, they actually blended quite well and I got through the gig.
I like the feel of the flats on the higher strings, and the little bit extra presence the rounds give on the low strings. I just might keep it that way for awhile until I get a new set of Fender Flats 9050CL.
BTW: go over to the DB side of the forum. They have been mixing and matching since there was anything besides gut. On my DB, for example, I have a Spirocore 3885.5W E string, Bel Canto A & D, and a D'Addario Prelude G, all to get more definition down low, good round tone in the middle, and less twang up top.