I'm looking to add a card scanner door lock to an existing door that currently only functions with a key and a simple pull handle. It has a mortise-style lock so the backset on the door (to the existing key) is only approximately 1". There is room next to the door along the frame so would it make sense to install another mortise-style lock into the frame and place the scanner on the frame? The door would then have, essentially, two locks but I'm not sure if there's a "best practice" in this case. If anyone has experience with these sorts of locks I'd appreciate some advice on suitable options and ease of installation.
They've done the exact modification that you're proposing (placing it next to the door, rather than in the door) at my place of work.
What you're looking for is called an 'electronic strike'.
You cut out the area around the existing strike plate, and wire in the electronic strike. You'll need to feed it power, and a signal from whatever you have to grant access. (in our case, the card reader, plus a motion detector so it'll open from the inside automatically; you don't need the motion detector if you still have a functioning door handle from the inside)
A quick search online suggests that they run $100-500 (US$), but you'll probably want to contact the manufacturer of the card reader to ensure you get something that's compatible.
As for 'best practice', much of it comes down to cost vs. what you're trying to protect.
The advantage of the electronic strike is that it can be hard-wired for power (no batteries to replace every few months), and you don't have to worry about cable bundles through the door or wireless signals (for auth & logging) that can be intercepted or interfered with.
If you're only protecting a single door (so don't have a centralized authentication server), don't need to log each person individually, and don't mind changing out batteries once in while, you might try looking at the locks they use for hotel room doors that replace out the existing handle + lock mechanism.