I don't think that is a bad idea at all.
Since Innova came out with a new large breed senior food I am thinking about mixing it with their (Nanook's & Raustyk's) food, which is Nutro Natural Choice Large Breed Weight Management (why do they make some of the names so long?LOL)

I just love the ingredients of other foods but the glucosamine & chondrotin levels in the NNCLBWM is just what I need for my two. They already get other supplements so I really didn't want to add 2 more when they can get it in their food.

Granted if I mix the 2 they won't be getting as much of the glucosamine & chondrotin, but since the Innova has better ingredients (IMO) I think it would be a nice balance.

My ferrets get a mix of foods & they do wonderful on it.

I never even thought about it for the sole purpose of making a better more expensive brand last longer, as long as all the brands are decent I think its a great idea!

Kaige seems to get bored of a food after about 2 weeks on it so this may be a great idea for him as well.