At the point where it's necessary to split the party in the Highpool Tunnels, I addressed this by using spc to leave formation movement mode and box-selecting all but one character to move. Reselecting the main party every time I have to switch perspective to the character left behind and back is annoying and error-prone, however. Is there a better way to split the party, possibly in a more persistent fashion like Wasteland used?
No, party splitting (like in Wasteland 1) is not a feature of Wasteland 2. The game doesn't seem to provide you with assignable hotkeys for party selection either (similar to how you select particular groups of troops in RTS games, e.g. Ctrl + 1-9).
You're pretty much stuck with manually selecting the characters you want to order around. I recommend moving the camera and using box selection (drag the mouse cursor).