If you want to format floppies with an Amiga file system on a PC then you need a hardware solution such as the Catweasel. Otherwise it is just not possible. (You can always do that on an Amiga though...)
If you want to format floppies with a PC file system (e.g. FAT16) that you can then use on both the PC and the Amiga (with CrossDOS) then you need either Double Density floppies or High Density floppies with a sellotape on the second hole. It is simpler to just format them on the Amiga - if you have CrossDOS installed simply select to format the PC0: (instead of DF0:).
