Yeg, Numpo and 10W
It would be of interest to know if you are employing an element of radar separation / monitoring in the equation at the time the second aircraft passes the fix - either UK environment or US side of the Atlantic - which might make what you describe acceptable to the receiving authority. However, if we are talking application of the basic purely procedural 10 min longitudinal (or 15 min in some parts of the world), then I stick by the previous post (Good on yer, Karrank).
To expand the point, consider the aircraft at FL 80, a Cherokee (groundspeed 1 mile per minute) followed by a TriStar (groundspeed 8 miles per minute) - same estimates. Are you seriously suggesting that separation exists until 1220 when the Tristar passes the fix???