if anyone is interested, yes, you can mount the monitors in portrait mode by using the included vesa plate.
All you need to do is attach the vesa plate in the orientation required and attach.
The only issue is it adds about 5mm so it is not flush if you align the monitors perfectly flat horizontally.
But if you use it like me and have a slight inward angle to the portrait monitor it is a non issue.
ps, if you need it flush, you can always mount both monitors with the vesa plate