Arrival Hongkong Airport/ visa free -> take ferry/train from Hongkong to Shenzhen want to stay

2 German colleagues/German passports will arrive at Hongkong Airport, stay at Hongkong 3-4 days (conference); then take ferry/train in the morning from Hongkong to Shenzhen; want to stay in Shenzhen for some 4-6hours max.(without hotel); and fly futher on from Shenzhen to Singapur in the late evening. Are they within the 72h hours free visa - at all >I am quite unsure /afraid that not .....

Thanks for your reply.