Evaluate frequency reponse of cascaded filter with bitwidth truncation in matlab
As bit truncation is not a linear operation, there's strictly speaking no frequency response, as the overall system isn't linear.
But: assuming somewhat benign input, I'd simply model the quantization noise as uncorrelated (uff, I know that's a big jump!)
linear system H1 -&gt; + quantization noise -&gt; linear system H2 -&gt; + quantization noise
or equivalently
linear system H1 -&gt; linear system H2 -&gt;  + colored quantization noise -&gt; + quantization noise
allowing you to combine H1 and H2 and then just add non-white noise of the appropriate variance.
Note that the variances of your two noise sources depend on the expected quantization error, which in itself depend on the bit-widening properties of H1, H2. You could math out and find that variance, or you could run these filters with enough test signal and simply measure the squared error between 12-bit truncated and untruncated output.
I'm not a great fixed-point filter designer myself, but I'm pretty convinced there's plenty of material out there helping you figure out optimal truncation.