My guy generally lets me know when he is wanting the second. Shortly after he swallows the first one all the way down and resets, he will look up and start flicking his tongue, searching for another mouse. Actually, the first time he did this was when I knew he would need two mice rather than one at each feeding. Once he is searching, I offer him the second mouse. Occasionally, if he is just getting ready to shed, he will eat the one and then go under his log, not searching for a second one (his appetite always goes down a bit right before shedding). Once he has shed though, he wants his mice IMMEDIATELY, and there better be at least two! 
Wendy