It is possible to drag a Connector from one Flow Chart Symbol to another but only within the same Model. It is not possible to drag a Connector from a Symbol in one Flow Chart Model to a Symbol in another, separate Flow Chart Model.
A work around would be to combine the two Flow Chart Models.
1. Click into one of the Flow Chart Models so that the gray border appears around the edge and then click on any Symbol in the Flow Chart so that it has selection handles on each corner of that Symbol (make sure your cursor is not blinking in the name field).
2. Choose Select All so that all the Symbols in the Flow Chart Model are highlighted.
3. Choose Copy from the Edit menu.
4. Click into the second Flow Chart so that the gray border is showing (again, make sure your cursor is not blinking in the name field).
5. Choose Paste from the Edit menu. The first Flow Chart may paste over the second Flow Chart so you will want to drag it down or to the right (the gray border will automatically enlarge).
6. Finally, draw a Connector line between the last Symbol of the first Flow Chart and the first Symbol of the Second Flow Chart.