If both the full SCC and Shorter SCC is used under an ECC Main Option C or A, it is understood that the Project Manager can use one or the other to make his own assessment and not both together?
The SCC should not be used under Option A. Option A should use the Shorter SCC for assessing compensation events.
Under Option C the SSCC can be used by agreement for assessing compensation events. However clause 63.15 does allow the PM to use the SSCC to make his own assessment.
Therefore under Option C the PM could therefore use the SSCC for assessing one compensation event and the SCC for assessing another compensation event. The choice would dependant upon the complexity of the CE.
Just adding to Dave’s answer, the SSCC and the SCC should, in theory, come to the same result as they are generally the same (it is just the detail that has been removed from the SSCC).
In a general sense I would agree that you use one schedule not a mix and match, at least on each CE.