As mentioned you will struggle to find an instructor willing to train with inverted doubles. The easiest way is to learn how to reach them. There are very simple and effective stretching techniques that will enable most people to reach the valves with regular stretching. If done regularly, increases in reach can be huge!!!!!
Background
Most of the shoulder is left suspended from the back and neck via muscle connections, which is good news and bad news. The bad news is that if you allow your muscles to get tight, then your range of motion will decrease. The good news is that soft tissue responds good to stretching, and over time you can gain very large increases in ranges of motion. I have summarised 5 easy stretches that will increase your reach dramatically.
Hanging Shoulder Stretch
1. Place palms on a wall, shoulder width apart,
keeping the elbows straight.
2. Walk your feet backward as you lean your chest
towards the floor until stretch is achieved.
Hint: If you’re tall enough, you can use a doorway.
Pectoral Stretch (High/Mid/Low)
1. Place palm on wall or doorway. To stretch the lower pec, the hand
should be at head level. To stretch the mid pec, the hand should be
at chest level. To stretch the upper pec, the hand should be at
stomach level.
2. Rotate upper body away from the wall until stretch is achieved.
Posterior Deltoid/Rhomboid Stretch
1. Reach your right hand over your left shoulder.
2. Place your left hand behind your right elbow.
3. With right arm relaxed, press on the elbow until
stretch is achieved.
Latissimus Dorsi/Triceps Stretch
1. Reach your right hand behind your neck.
2. Place your left hand behind your right elbow.
You might have to reach around the back of your
head to get the best stretch.
Swimmers Stretch
1. Reach both arms behind your back.
2. Clasp hands while keeping arms as straight as
possible.
3. Press arms up as high as possible.
3. With right arm relaxed, press on the elbow until
stretch is achieved.