Is it possible to monitor several outputs (e.g. 1 and 2) of the same NETIO device?

Yes. Simply create a new rule, give it a new name (e.g. Watcher2), change the value of the output variable to 2 and set the required limit. Remember to rename the _G.timestamp global variable, e.g. to _G.timestamp2, in the whole script. The two rules will run in parallel.