Zebra finch incubating eggs?

i live in the uk its just turned spring as you all know and my zebra finches have paired up. (they are in aan avairy outside) they have made nests and one pair have laid 2 eggs, but ive noticed today they have started to incubate them, there is only 2 eggs. can zebra finches inubate before they have finished laying? and will she lay any more