Unless you make an arrangement otherwise, they'd be resigning from their current role and taking on the new one with the shorter contract period.

Hi Helen, I assume that if the employee moves into the other role (12 months), you intend to backfill her current 2yr one? The contracts should work independently of each other, therefore make it clear that the 12 month contract and therefore their employment, will end after 12 months (which by law happens automatically) however out of courtesy it helps to specify this in the contract, with no guarantee of moving into another role.

Thank you both - I thought as much but wanted to double check.