I've got a double chainring with an 11T difference
Really? On a Brommie? Tell me more, please.
Side effect of my quest for 160mm cranks and a 28T chainring, mostly:
Spa XD2 triple crankset, 118mm bottom bracket with 1mm drive-side spacer (the Brompton 119mm BB probably works without), 28T and 39T rings, 42T bash ring in the outer position so your trousers don't get oily. Foot'n'finger shifting.
8-speed hub, so front shifts are a pick-your-terrain sort of deal.
If you don't need such low gearing the XD2 compact double with a 113mm bottom bracket is the way to go. Or if you've got an old-style Brompton crankset, you can bodge a granny ring on the inside with a bit of work.
If you're happy to always shift to the big ring before folding, you can have a *much* bigger difference between chainrings. Don't ask me how much.
Front dérailleurs are possible, but require proper engineering. The fold complicates things. Most people running multiple chainrings on Brommies don't bother. Alternatively: Schlumph Speed Drive.