They dont have to be twin tube to have seperate bump and rebound adjustment, actually, I think most 2-4way adjustable shocks are monotube. Only twin tube 4 way adjustable shocks I know of (havn't looked into it that much) are made by Ohlins, namely the TT40/TT44 (discontinued, and got replaced by pretty much the same thing, but monotube, according to their website) and the TTX40. These aren't your typical twin tube shock thoughs, especially the TTX40.
What you have suggested is how most twin tube shocks work (bump valving in the foot-valve and rebound in the piston.)