I am about to buy a patio door and a side entry door. They are from different manufacturers; both come without handle trim (no lever, rosette, etc) but with locksets - one cylindrical and one mortise. I would like to use the same key to open both - can a locksmith re-key them to be alike? If so, would both have to be re-keyed or is it possible to re-key one to match the other?
As long as you can remove the cylinders, the locks can be rekeyed. However, if the locks are from different manufacturers (Schlage, kwikset, Yale, etc.), the cylinders or more may have to be replaced. When purchasing the doors, ask if you have the option of which brand lock is installed. Try to get locks from the same manufacturer, if possible. If you can, match the brand to the existing doors in the home, then you can have all the locks keyed to use a single key.
If the locks are both from the same manufacturer, they should be able to be keyed to use the same key. This could mean rekeying both (if it's a new home, and you're changing all the locks), or just rekeying one (adding a lock to an existing home).