Some great posts in this thread. I just posted in the Grandparent Shower thread about a baby shower that my friend Amy wants to throw for her cousin's wife, who is having her FOURTH child (first one with this husband). I won't rehash it but all the reasons Amy wants to throw one are things that would make me cringe if it were the pregnant woman saying them--been so long since the last baby (older kids are teens), can't afford to buy stuff themselves, family members would be giving gifts for the baby anyway, etc.. But Amy really, genuinely wants to host it, and if the GOH is okay with it, and Amy only invites people who are 100% okay with it, it's not like I'm going to call them rude. To me the last condition is the toughest to fulfill, though, if she's planning to invite all relevant family members just because they're family members, and not the subset who have actually expressed spontaneous interest in doing this themselves.
Personally I think no one is owed a shower, even a first one. First baby showers have become the norm in a lot of places, so okay. I really don't think anyone is owed one for a 2+ baby. I could see certain extreme circumstances that would make me more likely to participate in a 2+ baby shower, along the lines of "house burned down, lost everything." But then surely people would be getting together to give other things, for all members of the family; no need to call it a "baby shower." I like the idea of a party to welcome an adopted child to the family, but again, that doesn't really have to be called a "baby shower," or a "shower" of any kind, or any term that suggests gifts should be given.
Anything like different gender, long time since the last baby, unexpected multiple babies--no, IMO. That's not to say I wouldn't give baby #2+ a gift, I'm sure I would, I just don't like being asked for one (invited to a baby shower). And if a small group of people who know they are all 100% into the idea of throwing a 2+ shower, get together to do it--that's fine, I'm not going to condemn them, because everyone there is known to be 100% okay with it.
OP, I think your logic for not having a second shower is sound; and I agree with others, and yourself, that this is not so much about showers but about your DH feeling resentful about people not reciprocating the favors you've done for them. Maybe you guys need to talk about your expectations more in this area, and set some parameters--not score-keeping, but if you doing stuff for others has a ripple effect on him, maybe you both need to evaluate what you do and who you do it for. Maybe it would work for you guys to have some kind of time limit each week for doing favors, or discuss each opportunity more as it comes up instead of just assuming you should do it (if you are). It sounds like you have a healthy, charitable attitude, but that doesn't mean his attitude is wrong, just different; maybe he just needs more reassurance that you know your limits and really enjoy helping others, that that is kind of the "payback," and you would be sad if you couldn't do it.