I would strongly suggest you don’t put anything in the location when doing on-line check-in.
Whatever you select there gets put into the “special request” field on your reservation.
The TP request gets copied and pasted into the same field.
Therefore there is a very strong chance whichever is done last will overwrite the previous one. I could see the drop box selection being automatically added, whereas the TP one will have to be manually added.
Which means …
If you do on-line check-in first, you should be OK as long as the two don’t contradict.
If the TP request gets added first, the online will almost certainly over-write it. No bueno.
Edited to clarify this is my suggestion.