Riccardo-

If you have a 12 volt inverter of any kind, you might look at the primary
side of its circuit. If the transformer center tap is connected to 12
volts and it uses switching transistors to alternately connect the ends of
a transformer winding to ground, then you can connect rectifiers from the
ends of the transformer winding, to a filter capacitor for a plus 24 volt
supply.

When one end is grounded, the center is still plus 12 volts, so the
opposite end of the winding becomes plus 24.

Thanks Fred, but i would like the dc-dc inverter solution to be able to
operate the switch from any 12/13,8 vdc source.

73 de Riccardo IK5WQO