>>216385979
>By entering a personal union like they did with Lithuania
if that didn't hold with tiny catholic lithuania it wasn't going to work out with russia
>I guess the UK and Spain don’t exist…
sure but the parts making up those unions were far more similar to each other and it was generally the senior partner dictating terms for the junior partner
a better example for your case would be spain owning the netherlands
I don't doubt that it could happen under a skillful monarch, but I don't think it'd hold long term, the differences are too great