I would run with the lock as the main and use mouseover macros to control the healing. Beneift being the priest doesn't need to be looking at you to heal.

For example, something like this:

/cast [target=mouseover] flash heal

Will allow you to move your mouse over your locks party picture, push the button to heal your lock and still maintain your locks target. If you ever have more than one lock (I run 4 locks and a priest) then you'll need to have the priest as main (I find it easier anyway) but if it's just 2 characters then lock as main should work better.