I have a pi4 running on an ssd over usb3 with a usb3 dock that has 2x2TB drives for storage.

At the moment I have mainly music on one and mainly video on the other, with important stuff on both and elsewhere.

Is it sensible to combine 2x2TB hdd’s via usb3 dock into a 4TB filesystem/pool/volume/thing…and if so can I have tiered storage so if one drive fails the other will have a mirror of important stuff?