Deep cleaning for periodontitis in one sitting, is that possible?

Depends: It depends on how severe the periodontitis. If it's very severe, it will probably not be possible to complete a deep cleaning in one sitting. However, if it is moderate then it probably will be possible.

It is possible but..: I'm alway a bit suspicious about it all done at once. Many people say they have had a deep cleaning but... A true deep cleaning is usually done a side at a time, the area is numbed and costs more than a regular cleaning. Also of interest, if the whole mouth is done on the same day, many insurance companies won't pay for it. And it's a proven myth that doing the whole mouth at once is best.

Yes: Be careful though - sometimes your dental benefit provider will refuse to pay for treatment done in one sitting.