Shadow types are a little more complicated then opposite types. It's basically where you keep the two middle letters and flip the two outer letters. By doing this the cognitive functions are all the opposite introvert/extrovert types but in the same order. Lets use Entj for this example.
Entj cognitive functions:
Driver: (Te) Extroverted Thinking (Efficiency)
Co-Pilot: (Ni) Introverted Intuition (Perspectives)
10yr old: (Se) Extroverted Sensing (Sensation)
3yr old: (Fi) Introverted Feeling (Authenticity)
Intp cognitive functions:
Driver: (Ti) Introverted Thinking (Accuracy)
Co-Pilot: (Ne) Extroverted Intuition (Exploration)
10yr old: (Si) Introverted Sensing (Memory)
3yr old: (Fe) Extroverted Feeling (Harmony)
Notice how they both have the same order of thinking, intuition, sensing, and feeling but it's their extrovert/introvert flip. This is because the first letter was switched so the pattern isn't extrovert, introvert, extrovert, introvert it is instead introvert, extrovert, introvert, extrovert. The reason why all the functions are still in the same order is because the last letter was switched which if you read an earlier statement that might make a little more sense.
Enfp = Infj
Entp = Intj
Esfp = Isfj
Estp = Istj
Enfj = Infp
Entj = Intp
Esfj = Isfp
Estj = Istp
Now, relationships with your shadow type can be really good but really hard at the same time. It can be good because you don't share any of the same cognitive functions so it can help you see the world from another angle. But, you guys have none of the same cognitive functions so understanding each other can be a bit hard.