Adsense Policy on Contextual Ads
Adsense publishers seeking to monetise their web pages are often uncertain whether they can display other advertising networks when they already have Adsense units on the same pages.
It can be difficult to interpret the Adsense Program Policy if you are thinking of signing up for another networks such as Text Link Ads, but the guidance should put your mind at rest. The main thing is to make sure that any other contextual ads you display alongside Adsense ads have a different appearance. They should use a different colour set from your Adsense Ads.
Here is the official Google Adsense Advice:
"You are welcome to display Google ads on the same site or page as other third party advertisements provided that the formatting or colours of the third party ads is different enough from that 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 ask that you choose different colour schemes for the competing ads."