Probst can host both. They won't be filming at the same time.
Seems they are covering bases. They know Survivor can't last forever and as it fades another one (hopefully) comes to light.
According to another article posted here about SURVIVOR (several exerpts):
Mr. Burnett and Mr. Probst have committed to the 19th and 20th seasons, but CBS has ordered the series through only the 18th.
Mr. Moonves said, however, that he expected to order more. “It’s not going away anytime soon,” he said.
CBS would be foolish to let “Survivor” retire, at least until it comes up with something that will deliver higher ratings in the time slot. That something could well be a new reality program.
But Mr. Probst was clear-eyed in a conversation at the offices of Mark Burnett Productions as he discussed the ratings in terms of the inevitable life cycle of television. “They will continue to dissipate until we’re off the air. Just like every other show,” he said.
A glance at this year’s prime-time schedule confirms that formats like that of “Survivor” are no longer the sole staple of reality TV...
The task for Mr. Burnett’s staff, then, isn’t to expand the “Survivor” brand. It’s to manage the decline.