Well, T-1000 had touched T-800 a lot of times.
Both are cyborgs - not as easy to guess the trick as it was with John's stepmom and Sarah, where T-1000 failed portraying the human behavior and stuff.
Is it the greater volume issue? But what about the floor and the guy in the asylum - greater volume as well?
Is it the complex systems issue? But it needed only T-800 appearence, not the mechanism.
Is it an Austrian accent issue? But sorry my English - not so perfect as well))
UPD: ok, "larger people restriction" and "the original T-800 is always around John" facts make sense.
What about pretending T-800 has lost its arms (like it lost one) or legs (like another T-800 did in the first movie) after disabling the original defender in the last fight? John will run to help, like he did in the final.
It would be only a part of the thing it had touched, but it's something like policeman's clothes-only and all the bugs after the nitrogen. By the way, these bugs could intercept my plan, hmm...
Certainly, I'm not that serious about it, just a little "I would"-style fantasy)