I've travelled two out of the three several times.

Two person rooms are available on most services. Ask for 高级软卧 (gaoji ruanwo) although there are also other names.

The Hong Kong to Beijing run is about 24 hours, giving you the late afternoon of one day and all the daylight hours of the next to view the countryside (haze permitting). The other two are overnights, so don't take too much time out of your schedule.