Examine the *reason* a second shower is against etiquette. It's rude because (a) people sometimes do it to be greedy and get more "stuff," (b) your friends are expected to contribute *again* financially in honor of your procreation, and (c) a shower is a celebration of becoming a mother, which doesn't apply to second and subsequent kids.
From what you said, I don't think any of these reasons really apply. For (a), I'm guessing you didn't have a lot of baby things before. You said you don't need much now, so there isn't the expectation that they will buy you something expensive - but heaven knows there are always extra things it's nice to have with an infant in the house (diapers, outfits that haven't been burped on by child #1, baby toys that aren't slobbered on by the dog, etc.) and the attendees should be able to buy you something inexpensive if they wish. For (b), it sounds like your life and your friends have changed a lot, so the people invited to this shower weren't involved for baby #1. And for (c), it really depends on your situation. Some people might see you as being a mother already so what's the point, while others would like to celebrate your emergence into the mom role in a traditional family setting - married, out of college, and presumably with more resources to care for the children. If you feel your life has changed since baby #1, I wouldn't begrudge you a celebration of that.
I'm sure some people will be snarky (or silently disapproving), but it sounds like you're ready to make the transition between "having a baby" and "being a mom." And that's something to be excited about.