[ZenGum]

Ibram, interesting thoughts. Arguing that a scenario is impossible is a reasonable response.

Nevertheless, I disagree.
I imagine some combination of hormones and pheromones and physiological response to suckling could well provide a possible mechanism to stimulate lactation, and behaviour guided by imprinting could limit this to direct offspring.

Would it have an evolutionary advantage? Hard to say, in lots of situations it probably wouldn't, but we only need a few situations in which it did, for it to be preserved.

I get your point about relative investments, but that raises the issue of sexual selection, and females would have reason to prefer partners who showed more promise as providers for the young.

I can imagine it being useful in mostly monogamous species by reducing the burden on a single parent and allowing bigger broods, leading to faster population growth. Mutual suckling could be pleasurable and reinforce pair bonds.

I can imagine it being beneficial for tribe/herd species in both sharing nursing burden and reinforcing group cohesion.

I think it is possible and could well be selected for. I mean, it is less useless than the peacock's tail.