Its All Schizoid!
If No.6 is quite literally No.1, then who is who is on the telephone when No.2 calls, or more often than not is called by No.1? On explanation could be that No.1/6 is suffering from schizophrenia and so at times behaves as No.1 but at other times he believes he is No.6.
However this theory hinges on whether or not No.6 is physically in No.2's office at the same time No.2 is on the telephone speaking with No.1. and in fact this occurs towards the end of the ‘Hammer Into Anvil’ episode, when No.2 is reporting himself as being a breakdown in control. So subsequently this scene blows this theory out of the water!
As No.2 once said "Back to the beginning!"
I'm Afraid Of Nothing!
If that's so, why does No.2 hide behind a pair of solid steel doors? Presumably they are blast proof! But apparently not No.6 proof, as you can observe how easily No.6 pulls the steel doors apart during Many Happy Returns - and using only his bare hands, presumably because the power is off he is able to do this. And how do we know the power is off, because the pair of steel doors did not open automatically for him on that particular morning, nor could No.6 boil his kettle back in the kitchen of his cottage.
A Sense Of Humour - Strong And Unimpaired
"Top of his class in woodwork!" During ‘The Chimes of Big Ben’ No.2 tries to encourage No.6 to "Join in." To which No.6 responds "I'll make you a handle for this door!" which I find very humourous, as does No.2 "Ha ha ha ha ha ha...." as no wooden handle would make the pair of steel doors work.
Be seeing you