afaik, the Duet is two Strisos with a custom ‘body’ which connects the two strisos via a usb hub, so you can buy one Striso, then later add the another and a body to make Duet.
you can see this here and here
so… this means, it does not need to merge the two midi streams, as a computer (etc) would see this as two different usb midi devices.
this is actually pretty handy, as it means its very easy in two have each hand/striso playing a different synth. and not have to cope with things like midi channel splitting.
pretty flexible for most use-cases.
but sure, a limitation, if your are sending to one device, you’d have to do some midi merging, with something that supports usb hubs, or can handle multiple usb devices. something like a teensy could probably do this without much coding.
guess in the continuum world, you’d just have to buy 2 micros 
overall, I guess its a pretty limited use-case… I doubt too many have a duet, and probably the separate hand = separate instrument works nicely for many.
edit: just seen @NothanUmber post.
ok, so that means the merge can be pretty simplistic, as you can just arrange first/last to voice conflict… just a matter of merging the two data streams together.