Going on 8 years I've had my personal phone connect to my iLoad van's inbuilt BT to play my personal media and calls. My Apple work phone stuff gets done through its own speaker phone which is a pain due to it's low volume.
The van has an android/apple carplay USB socket on the dash.
Is there a device I can connect to the USB so that both phones can connect simultaneously?
No. CarPlay/Android Auto won't work, it is single device only.
You should be able to have one device via CarPlay/Android Auto and then another via BT. My Wife's Hyundai Tucson allows that. I can have my phone in for Carplay and then if her phone rings, it'll be via BT.