Looks to me as if you need to change the stoker's right crankarm,
to one with a boss on the back side of the spider arms for a smaller 3rd chainring,
to drop down to a significantly smaller chainring .. like a 24t .. 74mm BCD.
that you can do with a single bike crank, chosen so you kep the crossover drive on the left side.

there are 'tripleizer' chainrings, [TA] the bolt circle for the smaller chainring
is made part of the 2nd chainring, so outer, 130, inner 74..
not sure how they handle the power of 4 legs.

If you can find a whole TA Zephyr Tandem crankset that would be a score,
the French Company stopped making them a few years back ..
advantage: the final drive crankarms were drilled with 3 sets,
a 110 outer ring circle, and 2 different bolt patterns for the granny gear.
either a 74,and a 56, the latter offers the potential to put a 20t sprocket on to further lower your rear ratio..

I'm guessing the rear hub is double threaded,
a freewheel cluster on your right, the Arai brake on your left.

not a freehub, so the 11t pedal down hill cog is out, limited to 13t

unless you have the wheel rebuilt with a current cassette tandem hub.
the one from Shimano retains the thread on the left for the drag brake..
and the 48hole drilling..

rebuilding the front hub with a Schmidt dynamo hub will be more reliable
lighting power than the Sanyo tire drive one that is there,
[and slips in the rain]
the front lamp would have the on/off switch..

oh , and there is probably no problem switching the final drive crank to your front crankset, chain is a lot longer, of course.
Its how tandems were set up in the 50's.
[no cross chaining issues because of the long distance
between the front and rear chain cogwheels]

but more common,is as you have, these days, short final chain. triple on stokers right side..