I think google TOS says no third party contextual advertisement should be displayed, so this may be violating it
Google does have that in the TOS - combining adsense with yahoo-ads is a violation of the TOS. So no, you are not allowed to combine them on the same page.
YPN doesent have problem with it as long as its just search and not the content ads. Adsense on other hand doesent want contextual ads by their search units. So the answer is its now allowed by Goolge, and it is allowed by Yahoo.