Why does Ubuntu only detect one USB LAN adapters at a time?
It sounds like a bug with USB ID generation for the devices. I've seen this with external hard drives across multiple platforms(windows too). I never resolved it. I hope you get an answer.
A workaround is to use multiple vlans on the interface and add a switch to your config just to split out the vlans...it's ugly, but it gets you where you need to be.
USB Lan adapters with other chipsets might work.
If you try it and it does, then it was a)inadequate driver or b)usbid generation.
If you try it and it doesn't work, then it's a usbnet subsystem bug(probably).