It won't work. DACs and ADCs usually have an error well beyond one LSB. All of the secondary DAC/ADC's accuracy is lost in the error of the primary DAC/ADC. There is a good reason ADCs / DACs get more expensive with increasing accuracy
Pretty raw presentation. Let's break this down -
As EEs, we all know about accuracy / temperature / span / drift errors. Generally a DAC is part of a feedback loop, so generally an ADC is
monitoring the output ... or maybe it's just visual, whatever. IF I add +1 STEP to my DAC, I will see +1 step output. Sure, it maybe 1-2+%
span error, or 5% absolute error, BUT it will STEP the output. IF my primary DAC can only produce OUTPUT steps of SAY 1.00V for the
range I need, then I HAVE to use a second DAC for FINE TRIM. Again, EVERY time I add +1 bit, I will see +1 step output, it's the law :-)
My feedback circuit will sort out accuracy errors, IF required. I don't think the OP is making a 1GHz sampling scope.
YES, I can sell my liver and buy a higher precision DAC, but that's NOT what the OP asked.
So, the basic rule is ... I put in +1 bit, I get out +1 step, otherwise it'd (the DAC) be called a DORK or something.