MCR allows you to perform a reconciliation on the contribution deduction amounts you provide for each employee.
What is the contribution process?
If you have two posts with two separate contributions, that means two separate contracts: C1/R1 and C2/R1 so they are treated separately. Therefore, they need separate contract identifiers.
If you have two separate posts, and you have only one pension contribution that covers all of your posts, then you know that you have contractable roles but only one contract.