Installing LED compatible dimmer switch (wiring question)
You bought the wrong dimmer -- time to return it and get something that's compatible with your application instead

Your dimmers are designed for ELV (Electronic Low Voltage) lighting transformer loads, which means they use something called reverse-phase dimming. In other words, they start the waveform at the zero-crossing then turn off in the middle of the half-cycle to control the power to the load, unlike regular dimmers, which control the turn-on point within the half-cycle and then turn off at the zero crossing point.

However, this means that your ELV dimmers need a reference to the mains neutral to work, which is something your box lacks.  Whoops!  So, you'll want to take these dimmers back and swap them for something else instead: a DVSCCL-253P should make an acceptable enough substitute given the constraints placed upon you (namely, the lack of a neutral), provided you're working with LED bulbs and not integrated fixtures.

Or, if you really need an ELV/reverse phase dimmer...

If you really need to use the existing dimmers, you can pull a /4 cable instead of the /3 (match the wire gauge of the existing cable) and use black for the always-hot, white for the neutral, and red and blue for the two dimmed hot legs.  That will put you in a good place for any future smartening you wish to do to these switches as well, too.