I use my Monk miner as a hybrid, so actually I quite often actively mine the same field and even the same rock as my drones. It doesn't cause any real problems. If you mine exactly the point of the rock your drones are currently mining they just stop and pick a new point to mine and keep going with very little slowdown (still it is best not to overlap because even a little slowdown is undesirable).
Supplementing your drones with your own active mining seems to actually improve efficiency if done right. In my experiences, because drones shoot at single-block targets and the majority of their array hits only by chance, they mine the first 3/4 of a rock pretty quickly, but as less remains of the rock it is often scattered and it takes them as much time to chew through the last 10-20% of the rock as it did the first 80-90%.
My technique for hybrid mining is to set the drones on a field, and go to work myself on a rock they are not shooting, keeping an eye on their progress. When they get 70-80% done with their rock, I swing around and help them finish, saving them a substantial amount of time, then return to the rock I was on if any are left at that point. It should also be possible to set your drones to work on a field and then move your own mother ship miner to an adjacent field and start work actively. I haven't tried since the big update, but I know before I could sick the drones on some rocks, then fly to the shop in the sector next door do some trading and work on my ship and the drones would continue working as long as I didn't go far or exit my core. So it might be possible for 2 players each with drones and miner motherships to parallel mine up to 4 fields at once or have their drones double team a field while the players melt roids in the surrounding sectors while keeping the drone sector visually loaded.
I am keen to try actually test multiple drone fleets mining the same field. My guess is that if they have different numbers of drones they should be fine because their members will take up different position layouts. I do worry that if both drone fleets had the same number of members they would probably try to take up the exact same positions around their target and spend the whole time jostling for position.
