Here are some excerpts from TA catalogs showing the bottom bracket spindles:




From this I gather the original spindle should be a TA 344 and the desired replacement a TA 374. What complicates this a bit is that both are available in two different overall lengths, differing on the non-drive side (NDS) part. That is not normally an issue however this is a tandem with NDS sync/timing rings, so you'd probably want to match the existing 17mm or 19mm dimension there. I'd guess that the existing spindle measures 19mm NDS so I don't think it will be an issue since all of the TA 374 spindles I've seen are the 123.5mm overall versions with the 19mm NDS length, like this one: T A 374 Bottom Bracket Spindle 123mm for Triple Crank | eBay

Note one of those charts has an even longer "Tandem" TA 375 spindle, but I believe that was for the tandem crankset version TA offered with the sync/timing rings on the drive-side.