Can we apply for 72 hr visa in Beijing if we have a 2hr layover in Guangzhou on way to Beijing?

We are flying from Vancouver to Beijing then to Cambodia. Our flight from Vancouver to Beijing has a 2 hr layover in Guangzhou .We want to apply for the 72hr transit in Beijing, can we do that? Do we have to go through imigration in Guangzhou or do we go through in Beijing.