I was always under the impression that this was against Googles TOS, though the other day I wrote them to ask if I could run Google search along side YPN and this is the response I recieved.
Hi,
Please note that AdSense publishers may display ads from other contextual
ad networks on the same page as Google ads only if the formatting or color
scheme of these ads is sufficiently different from the layout of the
Google ads. In other words, if you choose to place non-Google ads on the
same site or page as Google ads, it should always be clear to the user
that the ads are served by different advertising networks and that the
non-Google ads have no association with Google. If the formats are
naturally similar, we'd ask that you use different color schemes for the
competing ads.
Of course, we're constantly investing in our advertising technology, and
we believe that Google ads will monetize best for most publishers. Be
aware that the addition of third party ads to your site may cause fewer
users to click the Google ads on your site and thus your AdSense earnings
may decrease. We'd also recommend that publishers be careful not to place
excessive advertising on their sites, as doing so may result in fewer
repeat visitors.
For additional questions, I'd encourage you to visit the AdSense Help
Center (Google AdSense Help Center), our complete resource center
for all AdSense topics. Alternatively, feel free to post your question on
the forum just for AdSense publishers: the AdSense Help Group
(AdSense Help | Google Groups).
Sincerely,
Becky
The Google AdSense Team
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