If you combine the light of 1pc. 6500K 80CRI LED with 1pc. 6500K 95CRI LED what would be the overall CRI ?
Again, it depends on the specific LEDs, brands, optics, how they are driven and other things . There is no simple formula to calculate this, you would need to measure it with a device.
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any rough estimate like maybe (95 + 80)/2 = 87.5 
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My guess would be that it’s pretty close to average.
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I combined a 2000K light that opple said is 100 CRI and a 6000K light that opple said is 100CRI and it went down to 97 
CRI is not a linear functional and does not behave as one expects when taking averages. What it tries to capture is the similarity between a given spectrum and the blackbody spectrum that best approximates it.
For most purposes IRL a weighted average is an OK estimate when you mix two beams with different CRI and similar CCTs. If you are driving a 6500K 80CRI and 6500K 95CRI at the same power level (assuming the emitters can handle similar amounts of power), the resulting CRI will be likely closer to 85 than to 90, since the lower CRI emitter is more efficacious and makes more lumens.
There are lots of pathological counterexamples showing why averaging doesn’t actually work. The example above with 2000K/6000K mix is excellent: the resulting mix deviates too far from any blackbody radiator (too much red/blue and too little of wavelengths in between) that the CRI necessarily goes down. Mixing between CCTs introduces imbalance like that.
Another is a thought experiment: take 2 identical 100CRI lights, filter one so that it only emits wavelengths <500nm, and filter the other so that it only emits >500nm. Each one individually would have extremely low CRI (missing red or blue altogether) but combined they make 100CRI.
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