Here in lies the difference between some sim exercises and reality.
What is going to cause a double flame out at cruise altitude? No fuel would be the obvious answer. Ice maybe the next option.
Other than that I can't think of anything too obvious that would cause the double flame out to occur.
A mechanical problem is unlikely to occur on both engines at the same time - the statistical probability of that is so remote as to make it not worth considering. (I suppose you could add in oil leak due to a servicing error - both engines being done at the same time by the same person making the same error - as happened on a 737 a few years ago, but if I recall correctly the oil ran away fairly quickly and it didn't make it to cruise, let alone sit there at 35 grand).
So if it is fuel, then you have run out - in which case you won't get a relight. Maybe the pumps have failed - but again the chances of more than one independent mechanical device failing at the same time is remote.
The fuel could have gone waxy due to extremely low temps - in which case an immediate relight isn't going to happen.
It could be intake icing due to gross mishandling - extended period in icing conditions without selecting engine anti ice. Again an immediate relight isn't likely to be successful.
So, what am I getting at?
I am not sure that the focus should be on getting an engine running again as the first priority. My thinking would be to make sure that you stay alive and not go hypoxic, don't get too slow and manage the flight path such that the aeroplane is controlled and heading in a helpful direction.
737's that I have done acceptance tests on (both ex factory and lease return) have shown leak rates around the 1800-2000 ft per minute mark, so you are not too far off a cabin altitude above 10000 ft.
The point is - don't rush and have a think. There is plenty of time before you hit the ground, you have a little time before your cabin altitude exceeds 10000 ft, just manage the aeroplane and people in it and manage things that come up as they come up.