Not only you may, but must… But first things first:
As we are reaching out to publishers who cater to our client’s target market and have the potential of monetizing their websites through SelfMadeTravelAgent.com affiliate program, we’ve encountered a number of very similar replies within a pretty short period of time. Here is how one prospective affiliate replied to our invitation (text rephrased to omit sensitive information):
Thank you so much for your interest in [our website]. However, we have recently agreed to partner with another hotel and flight booking vendor through their affiliate program, so we are not open to any additional programs at this time. We do not want a conflict with the other affiliate program.
Interestingly enough, I addressed this very misconception (that once you’ve joined one affiliate program in a niche, you cannot or should not partner with conflicting advertisers in the niche) in my newest book, Quick Start Guide to Affiliate Marketing: Answers to the Questions You Should Be Asking. On page 20 I answered this question:
May I promote more than one merchant from the same niche?
Absolutely! As a free marketer, you are the one who chooses the merchants into which you are going to invest your effort. Merchants know this, and in the course of all my time in affiliate marketing, I have never seen a merchant who requests, not to mention requires, exclusivity.
Not only you may promote more than one advertiser in/from the same vertical, but you must do so. After all, how else will you know that you are earning the maximum possible revenue (permitted by your circumstances) from within the niche? Split-test analyzing such KPIs as banner/link clickthrough rate, conversion ratio(s), and ultimately EPC or your average earnings per 100 clicks.
Good luck monetizing your websites!