Neither Doncaster Knights nor Ealing Trailfinders will accept promotion to the Premiership if they win this season's Championship play-offs.
But the other clubs in the play-offs, London Irish and Yorkshire Carnegie, would do so.
Irish and Carnegie have met the Rugby Football Union's minimum standards to play in the top tier in 2017-18.
Preferring Championship rugby for next term, Doncaster, in fourth, and Ealing, in third, did not apply for that audit.
A statement from Doncaster - who would have gone up if they had beaten Bristol in the 2015-16 play-off final - said they currently do not believe it is in the club's best interests to be promoted.
"Having previously looked at promotion we feel it is simply unaffordable and out of our reach in the current structure and financial arrangement for professional rugby union in England," Knights president Tony De Mulder and chief executive David Ryall said.
"If and when the future of professional rugby union in England becomes clear, then the ambition of Doncaster Rugby Club remains as fervent as ever but we will not cripple ourselves chasing something that is not feasible at this time."
This year's semi-finals, which take place over two legs, start with Yorkshire's trip to Ealing on Friday, before Doncaster host London Irish on Sunday, 30 April.
London Irish finished top of the Championship table in the regular season, with Yorkshire Carnegie second.
From 2017-18, the Championship play-offs have been scrapped, meaning the league winners next term will be set for automatic promotion instead.