Plan 1: Take the Airport Bus 330 (airport bus line 1) from Shenzhen airport to get to Shenzhen Railway Station and then take the bullet train to go to Guangzhou. After you arrive at Guangzhou railway station, you can take the airport bus to get to Guangzhou airport.
Plan 2: Take the airport long-distance bus from Shenzhenzhen airport to Wanjiang Departure Hall in Dongguan directly. Then, take the airport bus from Wanjiang departure hall to Guangzhou airport.

It sounds that the second plan is more convenient. Both Shenzhen airport and Guangzhou airport buses go to Wanjiang departure hall to pick up their passengers, so I think wanjiang would be a good connection point.