I've never had this happen but people say the first one hatches at 18-19 day mark and the second egg hatches two days later. My last one was 12 hours apart
It depends on the pair. Some start incubating the first egg from the get go. Others tend to stand over it and do not start incubating until the second egg arrives. I also have eggs not hatch until the 23 day or even later. Ambient temperature also plays a factor I am thinking.