I asked my 2 undergraduate research supervisors on Friday, December 22nd. This is five and a half weeks before the deadlines (they would each be filling out 2 separate recommendation forms). The deadline is January 31st. I heard back from one supervisor about 10 minutes after I sent the email. However, I still haven’t heard back from the other supervisor. I am worried I sent it at an inappropriate time (too close to Christmas). Tomorrow will be exactly one more before the deadline. Did I wait too long to ask for the LORs? When should I follow up with the second potential writer?
No. You gave enough time for them. However you can't count the holidays as work days, they are coming back to work in the next days and then they will have approximately three weeks to write the letter.
I would send an email at the end of this week politely asking the second professor to confirm whether he will be able to write the letter or not. In the negative case you still have enough time to ask to another professor.