My husband got his left testi split in an accident in his teen yrs. They did and ultrasound and it was okay. But can this effect his fertility?

Unlikely. You only need one "good" testicle to maintain fertility so i don't think you have anything to worry about. One way to know is to do a semen analysis. If it's ok, then you have your answer. Good luck.
Possible, unlikely. A man is usually fertile with a single testicle thus even if the left does not work, he should still be ok. He could have poor sperm quality independent of the past trauma. You may want to get a semen analysis to get reassurance.