Powering 2 dc motors.
L293dne has 4 outputs. I assume 2 of them goes to one motor and the other 2 to another motor. 

I'm not sure why you want 18 V stepped down to 9 V, unless you are using 18 V batteries (a schematic would've helped). You can use 12 V batteries and step them down to 9 V. Or use 3x lithium cells (3.7×3 = 11.1 V) and step that down to 9 V. 

Or you can use solar cells together with some super capacitors (some tens of Farads @ 9 V).

You can also just use some 9 V batteries in parallel and get rid of a step down regulator all together. 

As long as you're feeding the L293dne with 9 V you'll be alright.