SOP's in my company is that the secondary is programmed for an early return to landing, normally your departure airfield.
Do I understand that you leave the Take off performance page on the secondary flight plan blank, but have filled in the airfield data for the landing?
If you decide to change flightplans BEFORE take off, then you now won't have any performance data for the take off. Best to pull over and think about what you want the nav kit to do before you decide to get airborne.
However, if you get airborne on the original primary flight plan, then it will have sequenced from take off to climb. Therefore if you need to return quickly, the landing data will still be in the secondary. Activate the secondary, activate the approach, and you should be all set up for a speedy return.