Just picked up a OnePlus 6 (considering a move back to Android from iOS) and it's working pretty well thus far. One of the SIM cards stops working during calls--which isn't surprising considering it's sold as a dual standby device--but at the same time I feel like they should if both networks support VoLTE. Of course, there's probably a technical reason why this can't be the case.
Also, this is probably not a surprise to anyone who has used recent OnePlus devices, but both SIM cards can apparently connect via LTE simultaneously according to a SIM card info tool I found on the Play Store. I'd love to try a Verizon SIM card in SIM2 at some point to confirm, though.