Oh, wait...I just noticed that you said you had one leaf turn yellow and then another turned yellow. Did they come from the same pseudobulb by any chance? If they were on the same bulb, I'd say that the plant is phasing that bulb out and that the bulb is dropping its foliage for the purpose of just being used as an energy storage vessel. If they came from different bulbs on the same plant, it could be that both bulbs are shedding and old leaf. When you need to worry is when you see simultaneous yellowing all over your plant at the same time...but that doesn't appear to be the case.
Oh, and after looking a bit longer, I would advise cutting the damaged leaf areas off that first plant. I don't like the way those spots look...they look like the damage may expand and start creeping into healthy tissue. And you don't want your plant to get an infection. The second plant is probably fine to leave alone. You may get more black spots, but I don't think the ones that are already there are going to get larger and start engulfing the leaf.
Hope that helped! Okay, now I'm going to bed...it's nearly 1 in the morning here! LOL