In theory there should be no such thing as a concurrent delay as one of these events will have occurred or been known about before the other. You then assess them in that order. You see what effect the first event had on planned Completion, and then assess the second event to see if it further affected planned Completion. If planned Completion was moved by one week as a result of the one that was not the Contractors fault then there is an entitlement to move Completion Date and this would be assessed in terms of both time and cost.
If the Contractor delay came first which moved planned Completion and the Employer delay had no further affect on planned Completion then the Contractor would be entitled to neither time or cost. If the Employer delay came first which moved planned Completion by two weeks and then the Contractor (concurrent!) delay came after and did not further affect planned completion then they would be entitled to two weeks in terms of time and cost. In effect the first delay creates float on the remaining work activities which could benefit either Party.
There is no event that in an unamended contract that would only ever assess time but not cost. That is a fundamental difference between NEC3 contracts and other (inferior!) forms of contract.
Would like to reinforce Glenn’s comment, " that there should no such thing as concurrent delay". All the lawyers I have dealt with in such situations dismiss concurrent delay as a theoretical one and that there is always a pre-dominant delay by one of the Parties.
With due respect the aim is to identify sequential delay i.e. which event (cause) occurred first (effect may be perceived as concurrent as to an extent it happens at the same time), rather than identify pre-dominant or dominant delay, which is a theory taken from contributory negligence which doesn’t readily apply to concurrency. Concurrent delay can occur but it is definitely a rarity, more so if you are working with a detailed schedule and attention to such matters, the latter as required by NEC.