But can you double when your legs are tired?

Iron Grip CrossFit – CrossFit

An excellent video from the Mainsite archive addressing some double under tips.
https://www.crossfit.com/exercisedemos/#modalPop

Warm-up

(No Measure)

30-20-10 seconds each:

Burpees

Jumping MB slams

High knees

Metcon

Metcon (Time)

For time: 10 min cap

5 Rope Climbs

or 10 min practice

Metcon

Metcon (Time)

3 Rounds For Time:

50 Walking lunges

50 Double unders