First, lets clear up some of the details you've given. Neither of the women are making claims of sexual assault. deRossi's claim is that he removed his pants during an audition and McCarthy's claim is that he insisted she remove her dress during an audition. Those are not claims of sexual assault, but rather sexual harassment. Neither of them mentioned a gun. The gun was mentioned by Julianna Margulies.
Seagal can sue both Portia and Jennifer for defamation, just as he can sue them for smashing up his private jet with a golf club (something nobody is claiming or was even possible). In the US, you can pretty much sue anybody for anything. Whether you can win is a different question.
Lawsuits are a civil matter and as such, at trial the winner is the one who has the preponderance of evidence. If Seagal's lawyer can convince a jury (or the judge if there's not a jury) that what these women claimed was not true and their claims were made with a malicious intent (malicious intent is required since Seagal is a public figure), then he would win his suit. If the womens' lawyers are able to convince the jury enough that either their claims are true or that they didn't have any malicious intent, then his suit would fail.
In Seagal's denial of McCarthy's claim he said that she did not even audition for Under Siege 2, so her story couldn't even be plausible. There are likely records of who did and who didn't audition for these big budget Hollywood movies. If his claim that she didn't even audition for the part is evidenced by the records, then he would likely have a pretty good case against her.
deRossi's claim might be a little harder to prove as the encounter did occur and it boils down to a he-said/she-said argument where several other women have made allegations along the same lines as the one she made.