....and let's face it, if you are resident in one EU country, commuting to another and are likely to go back home should your job finish or to use healthcare facilities or retire why should you pay SI on the basis of a notional base? If the centre of your life's affairs is in one country why should another state get the benefit of you paying, what is essentially, a tax for no benefit.
Just sounds like a massively over-complicated way of ensuring more jobs for government employees in both countries sorting out cross-border entitlements. Thank you Brussels.
It'll probably turn into a massive nause if either one of the countries ceases to remain within the EU too.