IF you have enough dough to charter your own!
OR-simetimes, perhaps, maybe, occasionally, there are cruises that do both countries (in TH thats alas always Laem Chabang-a disgrace to even get into), that may have some unwanted last minute beds left empty and maybe, sometimes, occasionally, there might be some travel-agent in the know and also having the ways of booking you into it-for just that sector. google for starcruises (from Malyasia)
With the recent start of direct bus BKk-PnPN and the overwhelming nr of direct PnPn-HCM buses, its just a 1 change (overnite needed) trip now.
#2 captainbkk has been a member since 16/2/2012. Posts: 472
Bus Bangkok - Phnom Penh, 11 hours
Bus Phnom Penh - HCMC, 6 hours