I think its a long-shot but worths a try... WI Stephen abdicates during 1140s-1150s (or dies if u prefer and leaves a will) and divides his Kingdom? Eustace IV takes England and William de Blois takes Jerusalem... Could this work?
Well, in your scenario (and I still don't see how Stephen and Matilda can be in the two different ends of Europe simultaneously), we'd have an inheritance which includes;
- Kingdom of England
- Kingdom of Jerusalem
- Duchy of Normandy
- County of Mortain
- County of Boulogne
There's no conceivable way that one man could hold that together. The only real question is which would be seen as most prestigious. Depending on when Stephen dies, we'd be looking either at Pope Innocent II (1130-43), who had previously expressed his support for Stephen, or one of his pro-French successors.
In the former scenario, Jerusalem would probably be emphasised as of greater importance (in a display of piety), while in the latter, England would be seen as a more established and powerful realm.
So, as long as we delay Stephen's death until, say, 1148, I see no reason why your scenario couldn't work. Of course, his wife is still kicking about until 1152 (assuming no butterflies), and she has the main claim on the crown of Jerusalem.
- Eustace I, King of England (114

, and sovereign of various French bits & pieces
- William I, King of Jerusalem (1152-)
And it's about now that the butterflies start to kick in, I'd imagine. Who do they marry? What's happening in Outremer?