how can i sync 2 different devices to the same computer without one trying to sync with the other?

Make sure that they have different names

Change the name of your iPhone, iPad, or iPod - Apple Support

Then just select what media yo want synced to each device. To make it easier turn off autosyncing so that you have to overtly start a sync. To turn off go iTunes>Preferences>Devices and check the box that says "Prevent iPods...".