hmm. if the toy follows that concept art, Onslaught is going to be a BIG deluxe. Maybe this is why the RID line's deluxes are so small...every inch shaved off an RID Cliffjumper gets added onto a FOC Onslaught?
Other than that, I can only think that maybe some pieces of the combined form come with each of the bots. Hands and feet are obvious, but perhaps the thighs are not built in to the centre, but the shins?
assembling a Neo-G1/TF:TM cast. Please PM if you have (or know of) the following at a reasonable price: Classics or Henkei Prowl, Astrotrain, Ramjet, Skywarp, Sunstreaker, Sideswipe, Blurr, Perceptor, GDO Wheelie and Springer, or 3rd Party Motobot Arcee, FP Springer, iGear Ratchet and Ironhide, City Commander (and/or Ultra Magnus), Starscream (deluxe) Coronation add-on, or Spike and Daniel in their Exo-Suits.