RAW is unclear, so this would be up to your DM
This is the sort of outside case that the rules do not cover, and thus such an occurrence would fall to your DM to make a ruling on.
For example, I would likely rule that nothing happens because, whilst merged with the druid, the bag of holding isn't really a bag of holding anymore, it's now part of a bug.
If the druid in bug form leaves the other bag of holding before reverting to original form, no problem. But what if the druid reverted to original form inside the other bag of holding? Would the two bags react to each other at that point, or would that only occur if someone tries to take the druid's bag back out of the other bag, or would nothing happen either way (the text does say "Placing a bag of holding inside ...", after all)? These are also consequences of such rulings that the DM would need to rule on should they come up in play.