Cascade DACs to get higher resolution
Simply put:  No.  It doesn't work that way.  In theory, yes.  In practice, not so much.

There are two main issues:  Noise, linearity, and timing.  

Nose is the easiest to understand.  Noise in the first DAC will be added to noise in the second, resulting in more noise.  

Next is linearity.  The DAC has more than just noise on the output.  Let's say that you have a 2 bit DAC, so it outputs 4 levels.  It should output 0%, 33%, 66%, and 100%.  Instead it might output 0%, 25%, 82%, and 100%.  This will make combining the output of the DACs together more difficult to do while increasing accuracy.

Timing is also difficult.  And by that, I mean getting the various DACs to output the correct data at exactly the same time.  If one DAC outputs it's new value slightly ahead of the 2nd DAC then you could end up with noise and spikes where you don't want any.

Use the correct DAC for your application and you will be much happier.