--multi-thread is for downloads only at the moment.
However box does support multipart uploads
--box-upload-cutoff SizeSuffix Cutoff for switching to multipart upload (>= 50 MiB) (default 50Mi)
So any file bigger than 50M will be uploaded in chunks and these chunks will be uploaded concurrently. See the box docs for more info.
If you turn on -vv you'll see a DEBUG message for each chunk so you can see how it works.