[Vwdiesel] Wiring a VW alt in tandem with another

[Travis .]

I almost have my JD L&G tractor wired up to run my VW alternator off the rear PTO (front PTO drives the snow blower). My question is on the exciter wire-does it need to have a diode on it or can it just be a switched power source that can be turned on and left on while running, or should it be a momentary switch to excite it and then turn off? I am pretty sure the rest should be easy just having the two alternators and batteries in parallel. The tractor alt has just enough power to run the PTO's and other functions to keep the tractor running so the VW alt is for the addition of the cab and its functions (wiper, heater, fan, lights, etc). I will have another battery in a ballast box behind the tractor (go figure-a vw rabbit battery). 

So-diode, momentary switch or just on/off switch for the the exciter wire. 

Travis