The only way you can mix 'n' match is between the UK and RoI, where a reciprocal agreement exists to accept each others Theory, Medical and Flight Training. Anywhere else, you have to have all three conducted in the State of Licence Issue.
Not because it states so in JARs but because that is how JARs have been interpreted by member states.
JARs actually state that it is possible to complete training in one state that has been commenced in another state - member states just choose to ignore this sub-para and get all protective of their own sovereignty.
Will it be any different under EASA?