I don't think the MR8 has overdub- per se.
You can't overdub on 1 track- what you can do is playback track1 while recording onto track 2.
Additional Ideas: Bouncing tracks, Auto Punch In, and Part Editing.
Onec you record track 1, 2, 3 and 4 you can bounce them together onto tracks 5 & 6... is that good enough?
Auto Punch in will play back the track, then at the designated punch in point switch to whatever you have on the input channel, record that, and then punch out and go back to what was on the track (pg 37 of the manual)
another idea is part editing. Its just like Auto Punch in, except you record the two tracks, then specifically cut and past a part from one track into another. (however you replace what you are pasting over!) (note: if you want to replace from track A to Track B at the same exact time, use the Copy-Paste part command).
So to recap: If you want to constructively combine sounds- you have to use two seperate tracks. Then you can bounce them together,
If you want to replace a part of 1 track, use the punch in or edit parts.
maybe I'm just used to reading crappy computer manuals, but so far I haven't any major issues with the manual... so tell me what exactly you want to do (Bounce, Punch In/Out, Edit) and I'm sure a bunch of people in this forum can help you out.