Can older control panels (e.g. Triomatic, Duomatic L Plus) be used for the remote indicator of the two-cylinder system DuoControl (CS) or the DuoComfort ?

Yes, with Duomatic and Triomatic you can use older control panels. Only with Duomatic L Plus it has to be replaced in a few steps. However, you can continue to use the cabling (from the gas cylinder box to the control panel). More information and a connection diagram for older cabling and control panels can be found in the installation instructions for the DuoC remote indicator.