Personally- I don't think they would be too big...but then again I love the look of stacking almost any ring. I think most people gravitate towards stacking smaller stones but I say go for it. If you can though grab some rings that are close in size to what you are looking at and try to see if you like the look that way.
Based on some of the pictures on her Etsy site, I think it would be possible to stack 2 of them. It's not a look I particularly like, but if you love it, go for it.