Different search engines conduct similar ad auctions simultaneously and advertisers have to choose in which search engine(s) to run their ad campaign. In this paper we discuss two models for a pair of simultaneous ad auctions, A and B: (i) singlecampaign advertisers, which participate in a single ad auction, and (ii) multi-campaign advertisers, which participate in both auctions. We prove the existence and uniqueness of a symmetric equilibrium in the first model. Moreover, when the click-through rates in A are point-wise higher than those in B, we prove that the expected revenue in A is greater than the expected revenue in B in this equilibrium. In contrast, we show that higher click-rates do not necessarily imply higher revenues in the second model.