Thanks for all the good information, the detail i'm not clear on is when there's a delay before the second seat fires.
The remaining crewmember is still seated in the cockpit when the adjacent seat launches if for only a brief time, surely the rocket efflux would affect him ?
Some responses state that a gun initially fires the seat before the rocket takes over however I thought this was only on much older seats and that newer models provide a far less violent ejection using a rocket.
Which takes me back to my original question !