it's pretty simple - you can have matching gear ratios (virtual fixed, but will feel like a really slack chain due to freewheel engagement), or a taller (smaller fw) gear on the left side. Here's why:

If the rear wheel is going at a constant 120 rpm, and your forward drive gear ratio is 2:1, you've gotta pedal at 60 rpm to engage the right-side freewheel. If your reverse-drive (left side) ratio is 3:1, you have to slow your pedaling cadence to 40 rpm to engage it.

*That gives you a 20 rpm window in which you're not engaging either freewheel.*

As the two ratios get closer, that window get's smaller until you're virtually fixed. If you were to go beyond equal ratios, the freewheels would lock against each other and the wheel wouldn't move.