I wouldn't cross sell my own sites. You will get more new members by X-selling with other sites. While you may only get $15-$20 a signup, the amount that they can send you makes it worth it imho.

The only way I would ever cross sell another site I had was if it was radically different from my other sites.

If i ran regular membership sites, I may cross sell a dvd download site I had, or a live cam site I ran, or... you get the idea...