NEC3 ECC: Which can Project Manager use - SCC or SSCC

If both the full SCC and Shorter SCC is used under an ECC main Option C or A, is it the case that the Project Manager can use one or the other to make his own assessment in each case and not both together?

The relevant clauses in the un-amended ECC specify which schedule is to be used:
Option A Clause 11.2 (22) specifies the Shorter SCC.
Option C Clause 11.2 (23) specifies the full SCC.

If you are using an amended contract that allows both the full and shorter SCC to be used, without specifying when each is used, I would suggest that either could be used, but not a mixture of the two. The main point for the Project Manager to consider is that he has to act in an impartial manner, so he should pick the schedule that most suits the work rather than the one that calculates the most favourable price.

For options A & B the shorter schedule of cost components is used by both Parties to calculate the Defined Cost elements within a compensation event. No option to use the full “schedule of cost components”.

For options C onwards, it is generally intended that the full schedule of cost components is used. However, it is worth pointing out that 63.15 allows by agreement the Contractor to use the shorter schedule, but more significantly the PM (without agreement) can use the shorter schedule (whenever they want). I have never quite understood the reason for this within the contract and could encourage some games buy the PM to use the one he thinks he can get to the lower number (which is not in the spirit of the contract but also the two schedules in theory should get broadly to the same number). It also means that Contractors need to be very careful that they fill in their shorter schedule percentages in within contract data part 2 for option C as well as the full schedule percentages - which I often see missed by both Parties.