Sure. So you take half the staff and put them on testing a second plane. Then each individual plane takes 10 years to test instead. (Remember that you don't gain person-hours for free just by shuffling people around.)

What would this accomplish? Instead of a total system time average of 10 years, you now get an average testing time of 10 years to which you still need to add the queuing time, so you're even worse off than before.

(The queueing time won't be five years with two servers in parallel, and I can't do the exact approximations in my head, but it'll be at least two years. In other words, by testing in parallel you worsen the cycle time from 20 years to at least 22 years.)

This is a good general rule: by taking on more work in parallel, you'll make the turnaround time worse. This is why lean consultants go on about limiting work-in-progress.

Also a call to learn some basic queuing theory! It comes in handy often.