Pulse measurement channel #1 maps to input #3, PM channel #2 maps to input #4.
You need to execute PMON 1 once only. Then connect your pulse signal to input #3, and whenever you execute
X = PULSEFREQUENCY(1)
the frequency of incoming pulses will be assigned to variable X.
The same goes for PMON 2 and PULSEFREQUENCY(2). However, remember that you should connect the pulses to input #4 in this case.
TBASIC is a generic language and PM1 and PM2 happen to share the same inputs terminals as input #3 and input #4 for T100MD+ PLC.