Just put them next to each other. That's it. It will create AND effect, since you need to pass both look-around before being able to match anything that comes after them.
In your case, it would be:
(?<!function )(?<!//)print
However, note that the regex above will return false positive, which causes more comments to be added than necessary. Demo.
For PCRE (used in PHP), look-behind assertion requires the pattern to be strictly fixed-length, so it is not possible to use look-behind assertion to check in all cases whether print
is being commented out or not to exclude it. @mpapec's answer gives one solution that is applicable for well-written code, and has better coverage than your regex with look-behind.