I just bought a 1988 Chris Craft Stinger. It does not have an external tie bar, just the inner. Do I need an external? Should I have both?

I'm in the same situation as you. I don't know if I need it or it's just a safety concern.

I would say having two is a good backup system.....if you have one and it ends up breaking, the drives will go all over the place.....hopefully not WOT!!! I never realy trusted the stock tie bar...just doesnt look beefy to me but hell....who knows. I personaly would rather have the nice stainless one on the outside.
If your concerned about it maybe have one made out of some nice stainless for the inside......or just keep an eye out on ebay...i see them going up sor sale every so often for about $100

Don't run two, only run the external. If you keep the internal tie bar and trim 1 drive up the way the geometry changes you loosen the steering arm or worse break a gimbal.

Well i had both on my 30' scarab and never had any problems......then again i never moved the drives around by themselves other then together all the time.....