If they are both in delay, they should have already been assessed in accordance with the contract. Which ever one came first should have been assessed as its impact upon planned Completion. If it moved and it was Contractor liability then there would be no change to Completion date. If it was Employer delay (i.e. a CE) then it would move Completion Date. The second event would then be assessed in terms of further impact to planned Completion, and if not Contractor liability would move Completion Date by the amount planned Completion has moved.
Once these two events have been assessed, now if the Employer de-scopes work whilst that may bring back planned Completion Date it will not move back Completion Date (as this can only ever be brought forward via agreed acceleration).
Delay Damages are then only liable if the Contractor exceeds Completion Date.