Can i use two different ad units one belongs to me and one of another person at a single time in my page.
it will display both. one of my sites originally was a free site, so my webhosting company has their own adsense ads on it. i didn't know that at first (neither did Google, since they approved my site first time) because my webhost rotates ads from different providers. one day i checked my page and i saw their adsense ads together with my ads, went back to read the TOS and confirmed that it was not allowed. removed my ads paid to get rid of my webhost ads. once i paid and my webhost removed their ads, i put mine back on.
If you want to run two, you could always just use some randomizer code to swap the IDs. it would effectively "split" the ads but only one id would appear on each page.
Worth pointing out that you can have two publisgers ads on ome site (providing on different pages). This can be useful if you are trying to attract other authors to contribute.