I'm not sure if I completely understand your question, but I'm assuming that PxtId is unique to each pair? If so, you can use a transaction to group those pairs into a single event, then do a count by UserId:
your search to isolate those two types of logs | transaction PxtId | chart count by UserId
This is assuming that both PxtID and UserId have been parsed as fields. You can do a similar search using a join on PxtId.