CJ, here's an update especially for you
I've experimented with every ring on hand. Best shifting and gear combo for us turns out to be the Stronglight
48T/32T with a 11-32 XT cassette. I wanted one more lower gear than what we previously had (48/30 w/11-28), to coax and reassure my stoker to start doing some longer climbs during our rides. With the 32T granny, that eliminated the need for any spacers and moved that ring closer to the 48T mounted on the middle position. Without the spacers the chain starts to chime on the big ring pins when in the 8th rear cog, but I do not intend to use cogs 8-10 from the granny anyway. Aligned as it is, the big ring works very well with cogs 2-10. So, this keeps the typical 16 out of 20 cogs to use without excessive crosschaining.
Info about other rings:
Due to the chainline I have setup which is based on the inner two chainring mounts on a standard triple and the 145mm rear spacing, neither a 50T or 52T work well enough to use. The 50T maybe just ok, but the FSA 52T is a definite no - the more severe chainline and tooth profiling on that ring is such that when the chain is in the smallest rear cog it wants to climb to the outside on the chainring. I'm 100% certain either of these rings would work on a wider spaced tandem crankset, or with a smaller rear frame spacing, with either mechanical or Di2 shifting.
This reinforces my future preference for a standard 135mm or a 142mm X12 rear spacing, as we need "skinny" stoker cranks and I absolutely will keep using electronic shifting. Perhaps if/when the eshifting is available with a triple FD, I might consider finding a skinny crossover setup (ie: DaVinci) at that time. At the time we had this frame built I had no intention of ever going Di2, but here we are now.
FWIW, both sets of FSA SL-K Light cranksets with BBs were a total of $550. That is close to half the cost of their equivalent (but wider) tandem set and could possibly recover the full cost if I ever decide to swap out to different cranks.