Producers...
Do not ask more than one vocalist at the same time to sing on your track. This is not a competition. If you feel a vocalist can work with your track, then you should stick with him or her. If he or she declines your track, do not tell the other vocalist that the previous vocalist declined your track, therefore, he or she is the "back-up" vocalist. If anything, you can also have two vocalists on one track. It is encouraged. Do not waste our vocalists' time by creating competition between them. Thank you.