I don't see the issue with it.

People have odd tastes. I could be into bears and leather as a prime turn-on BUT I have this thing for lumberjacks in jockstraps and if a site I TRUST also has a lumberjack site, I'll give it a try. I may like it so much that I drop my sub. to the bear site. However, I'm now a full time member of the lumberjack site.

I didn't know I liked rocky road ice cream until I tried it and I hardly ever eat chocolate anymore. But Haagen das still gets my money.

But the "same" content on two sites in a cross promotion could turn off a lot of people. It's like buying 2 copies of Runaway bride on dvd because you love it so much.