The only exception I give to second showers is if it's the father's first baby and it will be a shower for his family. Since they have yet to welcome you into motherhood, sure, let them. But if it's a shower for your friends and family who have already attended one, then I'd think it's tacky.
Here's a good rule of thumb for these kinds of questions: think of the guest list. Of the people who would be invited, how many of them have ever attended, thrown, or been the guest of honor at showers for subsequent children? If it hasn't been done, I wouldn't be the first!