yeah that & two easy wheels - not as stable as a rack but rolls pretty nicely now.
bafflingly, I also had to turn the handlebar catch upside down to stop spokes from catching on the hub gear indicator chain, but that's easy (once you discover that you need to do it!)
yeah I think you would need to double up... If you're not doing rack, though, this has been working well for me (at the suggestion of @sbbohr ...):
https://www.sjscycles.co.uk/brompton-spares-accessories/multis-mudguard-wheel-for-brompton-black-wheel-silver-bracket/