Basically you can't
The station that the train dispatches from is the station after the first node.
But what you can do is press F11 as soon as the simulator starts, and wait (as the train at the other station will dispatch automatically still) and then dispatch the other train manually, then set it back to Automatic mode by pressing F11 again (so it makes a cycle )
(iv done this on various of my Dual-station tracks like Big Thunder Mountain if u look at the exchange )
Rarwh