I just paddled my "Echo" double (24"x 18.5') alone. It should say
handles fine fore occasional paddling. Better than some plastic "tubs"
I've been in.


You just have to put small sandbag or chains in the front hatch to
weigh
it down a bit.

Echo is discontinued, though. You can find it used, I saw one on E-bay
a year or so ago for $1500.
Another small tandem was made by Lincoln kayaks and had same
dimensions.

All other tandems are made having outfitters in mind (rentals and
touring)
and are made excessively heavy and wide.

I don't know about convertible boats: the one with shifting seat has
cockpit that is too long (knee bracing?) and another with central
hatch will require
seat installation and two cockpit covers. Again none of them is narrow
enough.

I would prefer just to through weight in and wrestle it to the water.
I think it weighs 60lb which is another issue with tandems if you want
to paddle (and load/unload) alone.