As the magnetic disk is bigger...well the format is bigger too.
It is common to get 720Ko disk (using the 2 sides, so 80 tracks)
== Software's issues ==
Many Modern CPC users replace their old 3" by an external 3"1/2, adding a Disk drive A-B / B-A switcher (allowing to use the external disk Drive as if it were the internal one = Drive A).
Also a side switcher allows to use a 3"1/2 disk like a 3" disk... switching manually the sides as needed by good old 3" disk drives (yet a decent sofware can do it).
The side switcher and A-B drive's switchers are needed only if you use old softwares, as most of them couldn't really figure they were loaded from B drivers, or has no such feature as double sided drives.
They were designed for good old 3" drive so the 750 DD 3"1/2 is not implemented.
But modern softwares designers can implement this fairly more easily.
Orion Prime use the DD feature.
Rick Dangerous 128+ (1.1 addon) seems to allow the game to be loaded from the B drive.
== Softwares released on 3"1/2 disk==