I'm not exactly sure what you are talking about but are you meaning one testicle is retracting which might appear like it is 'inside' the other? Here is information to copy and paste on retracting testicles. https://www.mayoclinic.org/diseases-conditions/retractile-testicle/symptoms-causes/syc-20377197 There is a muscle there that can become overactive. It's called the cremaster muscle and there is something called the cremaster reflex. If the reflex is strong enough, the testicle can actually retract. If you think this matches to your situation, talk to your doctor about it.
More info would be useful.