Question on Proxmox Pools (I'm a bit of a noob)

I’ve added 2 external USBs of 2TB each to my Proxmox server and created a Resource Pool called USB_HDD containing both. I created an Ubuntu VM, but I can’t allocate all 4TB to it in one go - it only allows me to add each one as a separate SCSI device. When I start to install the OS it only allows the install onto one of the...