Thank you, thought so. Just to reclarify.
Wire Nest 2 to 1 on the boiler junction box, and Nest 3 to 2 on the boiler junction box?
(Boiler 1 went to old stat 2 and Boiler 2 went to old stat 3)
I can then use the redundant black and grey wire of the old stat to wire into nest T1 & T2 to wall...