This requires some manual picking rather than the 'just do it' option, but we picked the relevant settings, let it reboot so it could get exclusive access to the drives, and off it went, while bellinghwoman retired to bed.
It took me a little while to note that it was actually copying the first partition to a 932 GB target.
And it copied the second partition to a 932 GB target partition.
Oh, thought I, it screwed up, we'll have to retry once it's finished all of this.
And then once it had finished copying that second partition, it started resizing it back down to the size we'd asked for.
And once it had finished that, and there was space before that second partition, it resized the first partition all the way up to the size we'd asked for.
And it finally finished, leaving the right partition size after all.
I'd love to know just why on earth it didn't make the target partitions the right sizes in the first place.