They won't be active at the same time as a matter of course. But the UK will be able to now generate a full time capability - the ships will cover each others refit periods. Much like the Albion LPDs will be run. Clearly it would be a great idea for the RNR to train for these two capital ships to ensure that they could be regenerated quickly if required.
I guess it very roughly works like this:
Before SDR availability was 150%
After SDR it was 75%
After 2015 it may be 100%
As QE will also need to supply the UKs LPH capability, I personally think the U turn was worth it if it gets the UK that crucial 100%