The two quarterbacks have rotated with the first team throughout training camp as they battle for the starting job.
"Games matter. The games start Thursday," Joseph told reporters after Monday's practice in the Pat Bowlen Fieldhouse. "I want to get to the games so we can get to the bottom of this."
Siemian, 25, has the advantage in terms of experience, having started 14 games in 2016. He passed for 3,401 yards with 18 touchdowns and 10 interceptions.
The Broncos selected the 23-year-old Lynch with the 26th overall pick in the 2016 draft. He played in three games, passing for 497 yards with two touchdowns and one interception.