Does jainism forbid two souls after their human life from meeting in heaven strictest answer possible no or yes even if one soul has committed a sin of watching 18+ content even if done repeatedly say yes
No.
In Jainism, there is no absolute rule that two souls who had human lives are forbidden forever from meeting in heaven. Heavenly beings can interact and meet according to their karmas. Also, a sin such as repeatedly viewing 18+ content does not create a special cosmic law that blocks two souls from meeting in a heavenly state.
Strictly from Jain doctrine, the more accurate answer is no.