I really like this concept, and thank you for keeping your two characters relatively close to each other. it makes it very easy to keep track of them both because my eyes aren't darting back and forth constantly!
I was also kinda shocked by the sudden switch from sending out letters to gathering them. And it would be kinda fun to see if there was a way to connect the mechanic of running out of letters to disconnecting/reconnecting the bikes. IDK if it works for this scenario, but if the connection were somehow made of whatever you were picking up, it might make more sense.