Reply by Loren

The easy way is to rip the board down the center, flatten each
half, thickness it, then joint the edges and glue it back together.

Maybe that’s not so easy after all…. but it does get around the
limits of your jointer and prevent you from turning half the board into
shavings and making it too thin in the process.

David’s suggestion is a good one too – if the wood is stiff enough
to resist being pushed flat by the planer, you can remove a lot
of cupping, but usually not twist, with just the planer.