I am not sure. But I do not see any problem in this kind of activity. Take a look at their TOS to see if they specify something like that - I never saw like I said such kind of restriction. I'll give you a example of the other site - from the advertisers: Google Adwords do not restrict to make marketing of its Competitor Yahoo Overture using his network and Yahoo Overture do not mind that its Affiliates (like me) use its competitor Google Adwords Network to promote his Search Engine product. Its not a perfect example but I think that my case is a extreme compared to yours.
From the Chitika Faq... eMiniMalls service has no exclusivity conditions. You can choose to use eMiniMalls with any other ad service. I use Chitika with Terrabit(I know crap ads but great cpc) and I have no problems whatsoever.