In Mythic+ dungeons, a pally tank can help out with healing a bit in a pinch, but go oom really quickly. Plus, while they are tanking they are getting hit so don't heal very efficiently. And frankly, they could probably keep their health up best by continuing their tanking rotation going (like keeping Shield of the Righteous up) rather than stopping that to heal.

Once every 10 mins they can heal pretty well - with Lay on Hands - but obviously that's nothing you can rely on with consistency. If you are going to run any kind of challenging content, your pally tank will need a separate healer.