Converting +/-12VDC to +/-24VDC
I don't think you need a negative supply - just use a double-pole relay to reverse the polarity.



simulate this circuit &ndash; Schematic created using CircuitLab
You probably need a center-off switch to allow you to stop the motor wherever you want - or a separate off/on switch.
(You probably killed the DC-DC converter when you connected it with the wrong polarity.)
EDIT due to OP's comment that the controller outputs +12V or -12V...



simulate this circuit
When the controller outputs +12V, the upper relay will operate.  When it outputs -12V, the lower relay will operate.
EDIT 2  Changed the upper relay to DPDT to prevent short circuits should both relays somehow become operated at the same time.