Yeah... Following up on Rick's comments - changing the dimming to drop the voltage could work, but isn't a good idea. First, I do believe the MR16 is PWM - and the PWM is actually on the negative side (not that this necessarily makes a difference). But I believe you'll see 12 volts across the device around 50% of the time. If you put a large capacitor across the leads to your 5v load, everything might be okay.
But what happens if somehow, you ran a bad sequence into the MR16 - one that gave you 100% on this output? Poof. All the magic smoke will pour out of your 5v parts. A simple mistake could be catastrophic. That's really something we don't want to see!
And I'm not sure that you can really run the MR16 with 5v in - since it has a 5v regulator to drive its internal electronics, it probably needs a minimum of 6 or 7 volts in.
But you could probably run it at 6v, and depending on what your 5v devices are, you might be able to drive them to 6 volts without problem. And by adding a resistor or a power diode in series with your 5v devices (to drop the voltage a bit) you could certainly expect them to work when the MR16 is supplied with 6v.
But there's not really a good way to run both 12v and 5v devices from a single MR16. So, I'd recommend using two separate MR16's - one running at 6v and one at 12.
Another possibility, though, is that if you have two of the same 5v devices that you want to work in tandem (i.e. at the same time), you could connect them in series across one of the 12v MR16 outputs. When 2 of the 5v things are connected in series, each will see only half the total voltage - so at 12volts, each will only get 6 volts. Maybe that's within tolerance for the loads - but again, you could add a series resistor or a couple of power diodes to drop the excess voltage. (Note, if you're going to try this - these DO have to be pretty much the same 5v things - if one draws a lot more current than the other, that "halving the voltage" thing goes out the window).
Hope this helps.
--Dave
