I'd verify with Truetone that it will in fact give you 200mA at 9V from a 100mA 18V output.
I know voltage * current = power, but it's not quite so simple as that.
Their website only says 100mA max, which suggests 9V at 100mA:
https://truetone.com/1-spot-accessories/
V189 - 5.5 x 2.1mm plugs. Converts 18V to 9V on 1 SPOT Pro power supplies and most other brands. Handles up to 100mA.
I'd either use the 500mA output, or use a Y cable on two of the 100mA outputs. Then again, it might run fine on a 100mA output alone, depending on your power supply (which one is it?), but that could be risky depending on how gracefully the power supply handles an overload situation!