"This UTM-30LX unit offers a 270° range from 0.1 - 30m. I believe it's also what Alex Kushleyev has used. I don't know what power and weight allowance you have left, but at another 12V and 370g, I imagine it will be challenge to fit, let alone…"

"Since it looks like all of the processing is done on each unit's Pixhawk, could some Lidar or sonar based proximity sensing be incorporated?"

"Is this based entirely on GPS? If so, is its tolerance limited to the 1 meter or so accuracy of GPS?"