Both are arguably the best warriors of their generation.
Both faced hardships throughout their life.
Both had to fight for adharma even though they knew they were gonna loose.
Their birth was UN-usual and even their death.
Both were involved in a couple of crimes [which they did not intended to do] which were the sole reasons for their death. ( Bhishma destroying the life of amba and keeping quiet during the vastraharan , And Karna calling draupadi a W***e and killing of Abhimanyu).
Both were invincible in war until Krishna or external force intervened. (Bringing Sikhandi for bhishma , Taking Karna's Armour, the three curses and Krishna and kunti's efforts ).
Both were the rightful king of Hastinapur.
Correct me if am wrong.