It really depends on the situation and how you want to deal with it. If you think the first person is serious and you are prepared to "hold it" for a few hours while they make up their mind, then that is your choice. On the other, if you get the feeling the first person is tyre kicking or you don't want to run the risk of missing a sale, then offer it to the 2nd person.
But above all, just be honest and don't mislead anyone.