It's actually an upgraded version of the same engine, and Path of Exile 1 is getting a lot of the new tech backported.
It was originally supposed to replace Path of Exile 1, but now they are going to exist together.
2 will be more refined and slower, with a brand new story and 6 new acts. Should be a fun time.
I believe the acts themselves will be longer and more fleshed out but there's less of them.
6 acts in Path of Exile 2 instead of the 10 acts that PoE1 has.