It came as Mr Johnson insisted Scottish independence was "just not going to happen".
He made the comments during his first visit to Scotland since Mr Ross called on him to resign over the ‘partygate’ row.
The Prime Minister told the Scottish Daily Mail: "I think Douglas has done a very good job indeed of leading the Scottish Conservatives, and he secured back at the last election, as far as I can remember, about 100,000 more votes than any previous Scottish Conservative leader. He stopped the Scottish National Party from getting the outright majority that they thought they could get.
"Every day Douglas gets up and campaigns for the union of our country in a way that is absolutely passionate and I totally share.
"All I would say is there is far, far more that brings us together than separates us."
On independence, Mr Johnson said: "Just look at the experience of the last two years.
"Scotland and the whole of the UK has benefited massively from the might of the UK Exchequer, the UK Treasury, whether it is through furlough or any of the other things that we've done together.
"I think most people could see that it would be an economic disaster to split up our country. That is why it is just not going to happen."
Kirsten Oswald MP, the SNP’s deputy Westminster leader, said: “It is utterly absurd that Boris Johnson and Douglas Ross believe that they can both remain in office.
“How can Ross and the Scottish Tories possibly fight an election under a leader they think is unfit for office and should resign – not to forget that their Westminster bosses dismissed their Scottish Tory leader as a ‘lightweight’.
“It’s clear that Boris Johnson and Douglas Ross can’t both remain in post with any credibility.”
Ms Oswald added: “If Boris Johnson refuses to do the right thing and resign, then people will rightly question if they can take Douglas Ross seriously if he continues to limp on.”
The Prime Minister has been sent a legal questionnaire from Scotland Yard officers to complete, as they investigate whether he broke his own Covid laws.
The number of fines issued for alleged parties in Downing Street and across Whitehall will reportedly be made public by the Metropolitan Police.
Scotland Yard is also expected to publish its reasons for issuing any fixed penalty notices (FPNs), according to an advice sheet said to have been distributed to officials by the Cabinet Office.
However, the question-and-answer document reportedly says the names of those receiving FPNs – if such fines are imposed – will not be revealed, and their details will not be shared with the Civil Service.
It comes after Downing Street said on Monday Mr Johnson’s responses to his police questionnaire into gatherings at No 10 will not be made public.