Negotiations between the companies and Unifor come during a rebound in U.S. business jet flights, as more wealthy travelers look to fly on private aircraft during the COVID-19 pandemic.
Bombardier shares were down 2.08% in midday trading.
Unifor said it is bargaining for a combined 2,200 workers at the same Toronto manufacturing site for Bombardier's Global large-cabin business jets and De Havilland's Dash 8-400 turboprop aircraft.
"We will remain at the bargaining table with both companies as the strike action is ongoing," said Unifor national President Jerry Dias in a statement.
De Havilland announced in February it would no longer make Dash 8-400 aircraft at the Downsview site beyond confirmed orders due to weaker demand because of COVID-19.
Bombardier spokesman Mark Masluch said "the talks are continuing and we are focused on seeing the process through to an agreement."
De Havilland, which acquired the turboprop program from Bombardier in 2019, said in a statement that it would continue to meet "all customer needs."
A key sticking point for the union during talks with De Havilland is the future of the turboprop program it acquired from Bombardier.
Unifor said it is seeking a commitment to maintain prop production within a "reasonable radius" of Toronto if manufacturing resumes.
Bombardier announced Downsview's sale to a Canadian pension fund in 2018, although plane manufacturing continues at the site temporarily.
De Havilland said Unifor's position "is drastically out of step with the realities" facing its business and the hard-hit aviation sector, adding the company "will not rush" to a decision on the location of a future manufacturing site.
Key issues in the Bombardier talks include workers' pensions, the union said.
(Reporting By Allison Lampert; Editing by Marguerita Choy and Steve Orlofsky)
By Allison Lampert