After you get the timing chain worked out, you can try whether you like in phase, out-of-phase, or something in between. I prefer the back cranks 3 teeth behind the front cranks on my tandems as this helps smooth the weak spot in my stroke (as the stronger pedalling pilot, with one of my children as stoker) but it is close enough that I can put pedal at the top on the inside of a turn and know that the rear pedal will not strike. All of this you can try and see what you like best,but first lets get that chain on correctly.

First thing I would do is rotate the ecentric so that it is as close to the rear as possible, then make the chain as short as you can get over the chainrings. My guess is that this will mean that you need to remove a link. Then, rotate the ecentric until the chain tension only allows about a 6-10mm deflection in middle of the chain. Tighten the ecentric and rotate the cranks to see if there is another spot in the chain where it is much tighter or much looser. If so, you need to see if you can get the timing chainrings more centered on their cranks (I think sheldonbrown.com has a good section on some ways to do this). If the chain is much tighter at some point and you cannot centre the chainrings, you need to adjust the ecentric so the chain tension is correct at the tightest point.

Last note: often the bolts that hold the ecentric get really stuck and are hard to loosen after they have been left alone for a few years of riding in mixed weather. I recommend coating them in anti-sieze paste before you install them, and if there are through bolt holes use longer bolts that stick all the way through that you can put acorn nuts on to drive them out from the threaded end, rather than just relying on the hex socket in the head. I have had to drill out these bolts after the heads have rounded out and that is a very delicate job that involves a modicum of stress.

It looks beautiful and I am sure you will soon be enjoying the graceful power of a two-motored vintage ride.

-Will