Jeroen wrote:

Can I use one slider to control the volume of a part in the range 9-16
and
can I use another slider to control the volume of a part in the range 1-8
at the same time?

No. The Faders are used to control their associated Part 1-16

Can I use one slider to control the volume of more than one part at the same time?

The controller that can control the volume of more than one Part at the same time is called the "Super Knob".

For example:
Can I control the volume of the piano (4 parts) with slider 1
and
can I control the volume of an dx7 with slider 2?

A "dx7"? Are you talking out via MIDI, or did you mean an FM-X Part?

But either way, again no, the sliders are tasked with controlling Part volume 1-16 in two sets of eight slide controls. Assignable Knobs and/or the Super Knob can be routed to control the volume of multiple Parts in any combination, simultaneously turn one up while turning the other down or simply controlling both in any manner you desire.