RSX programs exist on top of AMSDOS to get up to 208 KB of usable space per side, by using 42 tracks with 10 sectors per track.
The problem with that is you don't have much gap between sectors anymore, and so your diskette becomes very sensitive to rotation speed.
The FDC controller can be used to control 80-tracks and/or double sided drives, though AMSDOS doesn't support such formats. To use 80-track drives and/or dual-head drives, you need to use another CPC DOS instead of AMSDOS. The most popular one is [[ParaDOS]]. It can handle up to 796 KB instead of 178 KB for AMSDOS and it supports 22 different disk file formats, including the three standard AMSDOS ones.