If Anglicans who sign the CCC are struggling to find a way to remain distinctively Anglican, one can only imagine the difficulties the Society of Saint Pius X will have maintaining anything like an identity worth maintaining. According to a story at Catholic.org, members of the schismatic group are not satisfied with BXVI's motu proprio allowing wider use of the so-called "Tridentine Rite" form of the Mass, because the guy "still supports the reforms of the Second Vatican Council".
So, before any kind of "reunification" can occur, apparently, VCII will have to be scrapped. In short, reunification is formally impossible, as far as these guys are concerned. If VCII were "scrapped" in the way required by SSPX, that would signal the end of the teaching authority of Ecumenical Councils and, hence, the Catholic Church as it has existed since the Council of Jerusalem. So either VCII is retained, in which case SSPX stays out; or VCII is scrapped, in which case SSPX is still out, because they will have reunified themselves with something other than the Roman Catholic Church.
So long, SSPX! Enjoy the bizarre world of Catholic Protestantism.