Hi,
Piano V1 sound different from V2. I think you'll be happy by just installing V2, as it sound so much better.
That said, then you can install both, and then uninstall one again, if you wish. Apart from the sound differnce, then i don't think you get any presets in V1 you don't get in V2, but V2 have more presets and more piano models.
You will not save much diskspace by not installing V1. But for the time being only V2 can open inside Analog Lab 3. Arturia have said they work to make it possible to open V1 inside Analog Lab 3 too. So at this point only installing Piano V2 might be most clean too.