You have to do whatever will work. Of course one party will be the more important of the two, so which parents party is the first or the real one and who gets the consolation party.
Two different parties at this point will set up the precedent and you may have to continue to have two different parties for every event in the future.
An alternative strategy will require a bit of strength of will on your part. Have you tried talking to both of them independently to determine if they are prepared to put the rest of the family before this negative behaviour toward each other. If you haven't, then do so and let them know how you feel and ask them to help you in your effort to keep the peace. Get a commitment from them to help keep the peace, if they will not give you one, then do not invite them to the party. If they play up on the day. Remind them in front of everybody about the commitment that they gave you before the party to help keep the peace. Express your distress with them not keeping their word. Apologise that this is not how you want family events to be and if they will not keep their word to help keep the peace, then you have no alternative except to ask them to leave.
If they apologise and behave then you allow them to stay otherwise, ask them to leave. If this process doesn't work with the baby shower, then you just try it again with the next event. Good Luck.