Best route I know of between Cowombat Flat and Cobberas 2 is to follow the other track a little east from the plane wreck, and pick up the main open lead running south east and then south, and then brumby pads up the spur before swinging back more westerly towards the northern end of the mountain. Cobberas 2 high point is at the southern end of the peak, with a northern plateau, a lovely snowgum studded saddle (good camping), and some nice bouldery bits in between. Once you're up on the plateau or saddle, the route south is pretty obvious (unless it's foggy). But the route from the other side, via Cowombat Track, is probably shorter if you just want to get up to the summit.
The red line is part of the circuit I did from the Playground - Cobberas 1, Middle, Cleft, Moscow, Cobberas 2 and down to the Flat on the first day. A long day but I'm *&%$#! slow. The blue line is a creek gully that provides a different ascent option - easy to follow but scrubbier.
When you say two days and one night for the sidetrip, did you mean the whole thing including the Pilot etc, or just the Cobberas section? The Little Pilot/Pilot bit is easy. The lower part, including the flatter ground to the Murray 'source' and Forest Hill, has a lot of regrowth and fallen timber and could be quite slow. Now, the option shown above is easier to pick up from Forest Hill than going all the way around to Copperhead Creek but, timewise . . .
. . . you could probably do the whole thing and get back onto the AAWT in two days, camping up near Cobberas 2 or Moscow,
if you don't get held up too much by the yukky stuff around Forest Hill . . .