I have two clean out drain side by side outside one has a cap the other one looks llike the cap has been broken off. Do they both need caps and does one need to be vented?

Both need caps, for sure... don't want anything to fall into the drain!

It is a health hazard to have open sewer by living space. As Mark suggested, cap them both off.

Milo

Agree, both need to be capped, can you post a picture, are they PVC or what?