Here’s a couple of question and answer pages about doing this kind of thing:
A couple of quotes:
Combining two DACs in parallel for current summing would in theory lead to better resolution - if the device-to-device gain mismatches and clock skews could be calibrated out. But in practice the device to device mismatch in clock propageation would be an issue and precisely controlling the relative gain between two separate devices would make the proposition be not really feasible.
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.
My limited experience with MCP4822 (12 bit) is that resolution is not as much of a limit on accuracy/precision as nonlinearity. Integral nonlinearity can be up to ±12 LSB. Combine two DACs and the nonlinearity in the high order DAC is effectively up to ±12x4096 = ±49152 LSB! If I’m thinking about this right and I may not be… but the point is it’d be nonlinear at a much, much larger scale than a real 24 bit DAC would be.