Hi:
I would not suggest to connect the devices in series like that. You will have a bad result. If the input imedance of the two devices in series is not exactly the same the power supply voltage will not split evenly. And of course the input impedance of devices like this is a complex characteristic and changes with the voltage drop across the devices.

I have used the DC-DC converters from Astrodyne...nice products. Check out their offerings at:
<big>www.astrodyne.com</big>

Mike Karas