Just noticed several companies running cross-brand ads at the moment:
KFC ads show they use real fresh chickens from Ingham and Steggles.
Dove's Refresh Your Body ads mentions Boost Juice as part of a complementary promotion campaign.
Here's another take on this approach: instead of striking up a deal with another business to partner on an ad, how about simply just saying wonderful things about a business you admire? This then becomes a congratulatory exercise, an act of goodwill and generosity; instead of being just a-n-o-t-h-e-r ad.
In a world where everyone is busy saying how great they are, this approach may well say more good things about your business than you could ever hope to achieve from any amount of self-hoisting hot copy.