These two were the most important leaders of the Parliamentarians during the early phase of the English Civil War. In fact, they were the only ones who could provide united Parliamentary leadership. Parliament quickly split into different squabbling groups after their deaths.
IOTL, one died due to cancer, the other died in battle. What if one of them (more likely Hampden), or both survived 1643 and lived at least until 1654, or even beyond that?
How would that affect the politics of the Parliament and the general developments of England/Britain during the rest of the war and beyond? How would the handling of Charles I proceed?