Again, I can tell you what it's not... and it's not a Schwinn (wrong building techniques & serial number sequence) or any other "brand" we typically think of when it comes to tandems, but the presence of a serial number suggests it's not some one-off job by a hobbyist. Serial numbers are typically used by folks who carry manufacturers and builders liability insurance and sell to consumers. And, while it looks a lot like a Jack Taylor, it's clearly not... wrong serial number location/sequence and building techniques.

Someone would need to send a photo to Dennis Bushnell to see if he might know who built it, as it reminds me of something he could have pulled-off, i.e., the nice and tidy TIG welds on a steel frame that may have been built when most tandem builders were doing fillet-brazed steel frames. The ovalized bottom tube also suggests someone knew something about designing tandems. However, don't be mislead by the use of a split-shell eccentric bottom bracket: Co-Motion even used those through the 90's as did several other excellent builders. It's just another way to skin a cat.

The component mix is interesting and does indeed at first blush remind me of what you'd find on an early Cannondale. Even the front fork looks very Cannondale-ish, but Tange oversized forks weren't unique to Cannondale and it may not be the original fork. There there are other subtle differences from the C'dale OEM spec components and, frankly... they simply represent the types of components that were available for tandems in the 90's. So, beyond the vintage of the components being a possible clue as to when the frame "may" have been made -- to include the 140mm rear spacing -- I'm not sure there's much else that it can reveal.

Again, hopefully someone will recognize it. I'm just throwing out my impressions, so I could be full-of-hooey with what I think I've been able to deduce from the clues provided.