What about the two heads respawning at the same time? That's definitely happening in my screenshot.
Also I did see other posts out there where people are saying the more currently decap'd heads there are, the faster they respawn (which I can see making some sense, given the massive damage increase especially for AOEs in that situation).
As a side note, someone needs to look at how Trunda's dmg is calculated. With the same conditions her damage from her A2 varies a ridiculous amount, like 300k-2,000k
Helmsmasher ensures that her A2 is NEVER the same conditions. Sometimes it triggers, sometimes it doesn't, and sometimes it triggers on the triggers, and sometimes it doesn't.