Simultaneous Wave

Is it possible to get a second wave to start while another wave is still spawning? I have a level where I want to spawn asteroids continuously throughout the entire level while waves of enemy ships attack. So, I put Wave 1 as a mixed pool of asteroids that spawns every so many seconds and subsequent waves to spawn various enemy ships. However, I can't get subsequent waves to start spawning while wave 1 is still active. If I have wave type as timed then the wave won't end until the time is up and if I set it to elimination then it won't end until all asteroids are destroyed. Either way the level won't continue to subsequent waves.

[EDIT]
If I have other spawners in the same wave then they can overlap, but then the entire level has to be a single wave and that causes other complications.