Rule 702.24b

If a permanent has multiple instances of cumulative upkeep, each triggers separately. However, the age counters are not connected to any particular ability; each cumulative upkeep ability will count the total number of age counters on the permanent at the time that ability resolves. Example: A creature has two instances of “Cumulative upkeep—Pay 1 life.” The creature has no age counters, and both cumulative upkeep abilities trigger. When the first ability resolves, the controller adds a counter and then chooses to pay 1 life. When the second ability resolves, the controller adds another counter and then chooses to pay an additional 2 life.