You will need to put a back-flow check valve in each line when you add the extra tank to prevent the larger tank from overfilling the smaller. Sherwood (an lpg equipment manufacter) makes a pv3866 bulkhead tee that has it built in along with a hydrostatic relief valve which is also required on all systems. You should be able to get it thur your fuel supplier (LP Dealer)
Make sure you are using wirebraid reinforced hose from the tanks to the lock-off not grill hose. Also I would recomend a 12 volt filter lock-off instead of the Impco VFF-30. I have had issues with the VFF30 not wanting to open due to low vaccumn

Regards
Jim