Operating with only one drive in a 2 bay device

currently i have My cloud mirror with 2x4TB drives in RAID 1. But i want to Change my backup concept. Additionaly i want to buy my cloud ex2 Ultra without drives and put it in another Location with only one drive from My Cloud Mirror in a bay and synchronise These two.
So my final configuration will be: My CLoud mirror with 1x4TB drive in it and My Cloud EX2 with 1x4TB drive in it in another localisation. The plan is: when 4TB will be full (both devices have the same data on it), i will buy two 4 TB drives and fill both devices with it. WIll it work?
Appreciate any help.

I would not recommend it. If you’re concerned about cost, just buy a single bay NAS, at least the same size, and copy the data over.