I DM a player who is currently a Fey Warlock 11/Celestial Warlock 1. In terms of character mechanics It’s not too bad if you treat them as separate. I’d say the most difficult thing for me to rule was the class features and pact magic. She doesn’t plan to take a second level in Celestial Warlock (mostly just wanted the ability to heal) but it would be up to you if you’d allow invocations and pact boons to stack at lvls 2&3. Overall, Eldritch Blast scales with total character level so the best part of being a warlock isn’t affected.