Trying to combine Two Measurements into one reading in Single Stat

I am am trying to combine Two Measurements into one reading in Single Stat.
The new KPI’s for PCF have two readings that take into account two different measurements. I am unable to get a reading. I have used the following.
M1 + M2
M1 merge M2
M1 JOIN M2
M1.M2
and none of them seem to work.
Any insight?