Can I use 1 Nest Thermostat with 2 boilers?
If the tstats get power from the boilers, the OP would have to be sure than only one of the boilers is supplying power to the Nest tstat. If the transformers were out of phase, you'd have a direct short. A dry contact relay for the 2nd boiler control would be safer, triggered by the call for heat on the first boiler.
But clearly the OP has 2 zones and 2 tstats are called for...best and safest approach. Like Freeman said, unless the heating performance and heat loss of the two spaces are identical, one space will be either over-heated or under-heated compared to the other.
BTW, what is currently controlling the 2nd boiler? A tstat in the outbuilding? An old fashioned manual one? If so, I really think the best solution would be to replace it.  It doesn't need to be a super expensive Nest type...programmable tstats for heating only shouldn't cost much more that $50-$60.  Probably the best and least expensive solution.