Kenz, I agree with those who suspect that Harley has caught something from Bayley...this is basically a very common scenario when a shelter kitty is introduced to a pet that is already in your home. When I brought home 8 week-old Ripley from the shelter, I noticed immediately that she was snuffly, sneezing, hacking, watery-eyed, etc. And, after her URI was treated, it was just in a matter of a day or two, when Fergus (8 months old at the time) started having the same symptoms!
Also, stress can compromise the feline immune system. With the loss of Davidson, and the newcomer, and the changing weather, etc., Harley's immunity was probably having a tough time fighting off whatever Bayley brought into the environment. That being said, you did NOT make a mistake by introducing a new cat immediately into the situation for Harley. If Bayley weren't there, there would be far less distraction for Harley in terms of the grieving process, and I can almost promise you that you'd be seeing a far more depressed and lonesome kitty. And once he's feeling up to snuff with some antibiotics (which are suprisingly inexpensive, most of the time), he'll be able to focus more on bonding with Bayley, and accepting him as a new companion.
Ripley and Fergus were put on Clavomox for their URI's, and another thing that works really well, is to give him a steam bath, which is sort of like what you do with a baby that has the crupe. Run your shower for about 5 minutes, until it gets HOT (I mean piping hot, with lots of steam!!!), and sit with Harley in your bathroom while the shower runs for about 20 minutes. The steam will moisten his throat, and help with any nasal congestion, and will also clear his chest a bit too. I steamed both of my kitties, and it helped a lot. Maybe just put Harley's laundry basket in there for him to snooze in, and sit there with him, so he's comfy. This wouldn't be bad for Bayley either.