The Onyx Filament can be used in all current Markforged 3D printers!
Printing large quantities of onyx changes the chemical composition of the print head in a way that is tailored to onyx printing. You are, of course, free to use one coil of onyx and one coil of nylon. However, you should choose a material, otherwise the print quality will be affected. If you want to continue using both materials, you should buy a second print head or printer.