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

No.

In Jainism, a soul’s karmas do affect its next birth and experiences, but watching 18+ content, even repeatedly, does not create a rule that two souls are permanently forbidden to meet in heaven.

Strictly speaking, Jainism also says:

• Heaven is not eternal
• Souls remain separate and individual
• No soul can stay in any worldly realm forever
• Only moksha is permanent freedom

So the strictest Jain answer is:

• No, Jainism does not forbid two souls from meeting in a heavenly state because of that sin.
• But there is also no permanent “together forever in heaven” guarantee in Jainism, because heavenly life itself is temporary.

If you want the most exact Jain conclusion in one line: No, but neither is heaven permanent.