Parties/lists can join national alliances. Alliances can run at most one candidate in each FPTP seat, but can run more than one PR list. The law explicitly forbids the so-called disjoint vote. That means that your FPTP vote and your PR vote must always go to the same alliance.
Interesting, one of the arguments we've been hearing from PR supporters here in Canada is no country that has ever adopted PR has ditched it but it seems Italy is heading that way although not to a complete FTFP system although I think AV is the best system followed by FTFP, but doubt you will see any European country that uses PR going to either despite the advantages.