Can’t remember when I last saw that word, synergy, anyways…
Firefox OS replaced #gaia and #b2g with a single #fxos and I think it worked out well for them.
Before that you had to monitor both, people who where around would answer questions more appropriate for the other channel.
For extensions I am aware of at least
#addons
#extdev
#jetpack
#webextensions
Would it not make sense to bundle that a bit, considering people will hopefully migrate from #jetpack to #webextensions?
There is also #apps, but there’s probably little overlap there.