Direct answers to your questions:
Medical schools will see both scores and weigh them as real scores. If you could pass off a score as low because it was for "practice", everyone would claim that for any score below a 30, they were just practicing. It's just not an acceptable reason, and yes, it will hurt you.
As for a void, yes it will keep the score from that test off of your record. Because the test will never be scored. So no one, yourself included, would see the score. I suppose you could take the test and void it at the end, but all you would have done is paid a huge amount of money just to a feel for the test day experience. You'd have no feedback on how you actually did.
I'm not sure how getting that experience (with no feedback) would help you study better, but I would recommend the same as everyone else. Buy practice tests. Then search or ask around the forum, and you can get a pretty good sense of what the experience is like. Then take the MCAT once.