Like 
@Ridgerunner , I 've done the research.  Calcium toxicity is a progressive disease.  Mixing one bag isn't going to kill them.  In all likelhood, if it hurts them at all, it will do so in ways that are clinically measurable but you will never observe.  Slightly smaller birds.  Less efficeint feed conversion.  Earlier onset of gout-like symptoms. and the older the bird is when it starts getting extra calcium, the less damaging it becomes (to a point).
I would not concern myself over mixing the two bags as a one off.  I would not make a regular habit of it with your hatchlings.
and FWIW, I do mix a 24% CP "grower" with a 16% CP "Layer" and feed it to all my adult birds, even though its "too much" calcium for my roos.  Call it educated risk taking.  I know the damage it causes - I can sometimes see the damage it causes when I bucher my own birds if they are male and old enough.  But I keep few of my roos past five months, and none past two years.  My hens can get to their third molt, but likely go to freezer camp at second - no evidence at all in them.
My hatchlings are on the "adult" mix generally between 8 and 12 weeks, though they are not yet adult - but most of their rapid growth is done, the effects are partially buffered by the way they have been fed up to that point...
TL;DR?  Time scale is important.  Age and Gender are important.  Brief periods of excess calcium will have effects you will likely never notice.  Go ahead, risk is low.
and if you want to get REALLY deep in the nutritional weeds?  P (Phosphorous/Phosphorus) buffers Calcium - so if your feed has relatively high levels of P (0.6%, 0.7%) the effects will be less than if its an 0.5% P blend.  Calcium DiPhosphate and DiCalcium Phosphate seem to be less damaging than Calcium Carbonate at identical rates of Ca inclusion...