Historical Rules · 1 May 2011
Rule 702.21b
Official source text from the 1 May 2011 Rules release
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 currently has no counters but 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.