As long as it's not on the same page, it's fine. Unless you're using the contextual ads in which case you can run both on the same page.
You can use AdSense alongside competing ads as long as the ad formats don't look confusingly similar. I use AdSense in combination with AdBrite inlines - together they're a winning combination and I earn about an extra 30% of what I do on AdSense alone.
You can use another ad network alongside adsense, but only if ads are not looking similar and the contextual is disabled (for the other ad network). You can find that in the TOS of the AdSense.