I just recently changed over from an analog hygrometer to a digital (which I should have done months ago, but we won't go there now). Also, changed from the "disc" hydrating device to the gel beads (allegedly holds at 75%). The Xikar meter I purchased gave me instructions to perform the salt test on it to get it manually calibrated (to 75%)the first time. I did so and it's now sitting in the humi with my sticks.

Here's background on my problem:
I bought a second digital hygrometer, the next afternoon for my second humi (which I just bought). Bought it from the same B&M, same brand, same look, except the one button on the unit was a different color (red, as opposed to green on the first one). Well, I didn't bother to do anything different setting this one up (performed the salt test). After doing the test, I went to manually calibrate it like I did the first one and it wouldn't automatically change to the 75% point that it was supposed to. Well, after searching around for the box and what instructions were in said box, this second one didn't give instuctions to do the test.

So....
1. Is this second hygrometer supposed to be one of those that calibrates on it's own?
2. Since I couldn't "manually" calibrate it, it reads about 10% lower than the one that I did maually calibrate. Right now, they sitting side by side in the same humi, one reads 69% and the other 59%. Obviously, one of them is lying. Maybe both.

What do I do?

Sounds like the second one you bought cannot be manually calibrated (slightly different model or something). I have two that cannot be calibrated. One is right on and one is out by 13%. I just live with it. As long as you are sure of the margin of error then you can just factor it into your reading.

Perform the salt test again on both hygrometers simultaneously (leave them overnight) and figure out your exact error margin. If you can live with one of them reading 10% low when you know it's fine then you're set. If not, take it back and tell them you want the model that can be manually calibrated.