the dc/dc converter you pointed to SHOULD work BUT may generate some(a lot??) of RFI. You may want to get one and test it.

I'm curious... why were you not satisfied with two LED strips in series?? Was it too dim? or too dificult to wire up that way?
From my perspecitive that really seems the very simplest way to solve your problem without the need for any more external components.

I would also breadboard up the circut and try the PWM via the ULN2003's and use the native 24vdc. (they can easily handle that voltage) If you keep the ON-duty cycle very short you should be ok.

Anyway... just trying to help you find the simplest solution.