Thanks for responding to my question.
I will try your suggestion to add the selvedge threads into the warp count (but still leave them 'floating' i.e. not through a heddle.
The suggestion about threading one side on 1 and the other on 2 is interesting but surely that would happen naturally in a draft for tabby? i.e. 1,2,1,2,1,2 etc. Even if the draft was on 4 shafts (1,2,3,4, etc.,) but weaving tabby.