How can one filter MULTIPLE processes in Activity Monitor?
This doesn't seem to be possible with Activity Monitor. Unfortunately top doesn't accept several pids to monitor either, so you have to revert to a half-manual (and rather unelegant) solution using Terminal:

while :; do
    clear
    top -l 1 -pid PID-TO-MONITOR | tail -1
    top -l 1 -pid ANOTHER-PID-TO-MONITOR | tail -1
    sleep 5
done


Given some basic bash skills you can wrap this into a shell script if needed often.

Or open several terminal windows and run top on specific processes in each of them. Might be kind of heavy on your CPU though...