I prefer the way Brompton resolves the need to carry stuff,
the bag is on a quick release frame clip on the headtube.
and the bag is pretty large..

an option is a version of the Ortlieb Downtown ,
but with a clip to fit it to the frame mounting block.,
and a couple waterproof extra pouches on the back.
accessory add on bags from their regular line-up.

But I cope with the 2 front panniers I strapped together, to work on my Bike Friday
as a single thing, though I have to carry them in my hand, rather than a shoulder strap.
because the strap stows nicely in the single bag ,
it would have to run across the front wheel with the pair.
potentially tangling with that wheel.