My Park wear gauge says its time for a new sync chain (.75), this will be the 1st change of the timing chain. I think I know what I want to do, but I'm fishing for opinions. The existing timing chain is an 8 speed chain while the main drive chain is a 9 speed. I'm thinking of replacing the timing chain with the same 9 speed chain as the drive chain so that speed links, spare links, etc. are the same (I'm thinking emergency roadside repair). The other thing I assume I have to do is buy 2 chains and cannibalize one for the extra length of the sync chain, right? I've heard about swapping the sync chain rings (front to back) to even the wear, but I'm not sure if I remember that correctly.

Get a Gates belt. No lubing required, lasts virtually forever, and no chain tattoos.

When I had a chain I went with the same speed for the reasons you mention. I also went with a higher quality chain with plating (Ultegra or DA) to avoid rusting since its life on the bike is a lot longer than the drive chain.

No reason not to go with a 9s chain if the price is comparable. As you point out you'll end up with some spare chain which might be useful one day.

As for rotating the rings front to back, that's a waste of time and effort. The wear rate of both timing rings is about equal since the chain load id the same on both. However chain load varies throughout the pedal cycle, and is highest when the cranks are horizontal. So what you might do, is advance the rings by one bolt position. (ideal is 1/4 turn but that's not an option). If you periodically advance the rings one position, you'll even out the wear through their lifespan.

You should read Sheldon Brown's article on this at Tandem Bicycle Synch Chains
Swapping the chain rings is a good idea. Also, if your rings have an even number of teeth, keep the teeth lined up with the chain plates as they are now.

I know this thread has been cold for a while, but I've got to ask; how do you this? Are you suggesting making sure the same chain link goes on the same sprocket tooth? I suppose that is possible, but it doesn't seem very practical. Am I misunderstanding the recommendation? I can see moving the front chain ring to the back and back to the front, and even rotating the chain rings one bolt every time you take the chain off to clean it, but keeping track of which link is on which tooth? That seems tough.

The Park chain tool checks a chain for wear for a chain that is shifted. Since you don't shift a sync chain, it can stand a lot more wear before it exhibits any wear symptoms. I think that we have over 13,000 miles on our old sync chain and it doesn't show any wear problems. If your sync chain get noisy or wants to skip, then it is probably time to change. However, if your sync chain shows problems, it is likely very worn and you would probably need to change your sync rings as well (they wear together).

I resisted the urge to replace the timing chain the last time the main chain was replaced. Probably about 3,000 miles on it now. Always ridden on the road and seldom in the wet. So, no reason to replace as long as it is behaving properly and looks ok?
Which isn't to say keeping it reasonably clean and lubed isn't important.

You can accomplish what is recommended on the Sheldon Brown site by just marking a tooth on each ring that engages the chain between the outer plates. If the rings have an even number of teeth, those teeth will only engage the chain between outer plates from then on. The idea is that this keeps half the teeth in better shape for longer.

You can put on many more thousand of miles on your sync chain.
Clean, relube and go!
Pedal on TWOgether!
Rudy/zonatandem