If you have trouble doing double pigeon, try it while sitting on a yoga blocks or a low stool. Another option is to do pigeon pose glute stretch as a preparation.
In this glute stretch you sit with your shins stacked on top of each other like firewood. For a deeper glute stretch align the top foot with bottom knee and the top knee with bottom foot. Looking down from above, position your legs so that the fronts of your shins are parallel.
So that you can get your knees down focus on pressing your feet down. First press your bottom foot down. Then press your top foot down.
It is actually more difficult to get the knees down while sitting upright. However, bending forwards is a bit more "sensational."
To stretch your glutes throug a full range of motion do this leg stretch first while laying back on your elbows.
Hold for a minute or so. Then from there sit up tall and hold. Then bend forwards.
To add weight to the glute stretch while bending forwards lift your hands off of the floor.
To gradually add more weight first reach your arms to the sides, then slowly reach them forwards.
Keep your neck long and your chest open as you do so.
To vary the stretch you can reach your ribcage and arms towards the side of the bottom foot. Then reach to the opposite side.
In all variations, work at pressing both knees down. (Or as mentioned above, focus on pressing the feet down so that the knees press down as a result.)
For a combined glute stretch and chest stretch you can do double pigeon while in lapasana shoulder stretch pose.