To me, it does not make much sense until Ghost adds multi-language support. I’ve chosen the option where I write articles in both languages using one instance, but mark them using different tags UA for Ukrainian and EN for English. So far so good.
You need to understand that all images you’ll need to host separately on each instance, or what is even worse - do a cross reference to images from one of the instances. It always an option to host images outside, but I definitile wouldn’t recommend it.
A lot of complexity in maintaining two instances adds as minimum the fact that you’ll need to reflect any changes in configuration/tags/layouts/styles/integrations and in each of your posts on your second instance.
Knowing today the amount of micro changes which I constantly doing when actively spend time with a my blog - I can say it should be hard to have two instances for different languages.
All above is only my humble experience and thoughts
Anyway I wish you a good luck with any option which you’ll prefer