Relactation, or possible tandem feeding, by non-bio parent

[Indadhanu]

NO we aren't pregnant with number 2... yet!

Shel is possibley, probably, most likely at this stage, going to be pregnant with the next little bubba. For physical and emotional reasons that I'm not going into right now (and which aren't the topic of this thread ), Shel doesn't think she'll be able to breastfeed. She willing to try of course, and go for as long as she possibly can, and I have a huge amount of respect for that, and she wants to at least give bub colostrum, but most likely it won't go much longer than 2-4 weeks (again, just a reminder, not really the topic of this thread). We've seen how good breastmilk is for Jazz, and we don't want to have to use formula so we want to be able to give the next bub this wonderful start in life (which most parents, BFing or FFing, can appreciate I'm sure, so don't get your knickers in a not, I don't mean any offence). So we're discussing me breastfeeding number #2 as well.

Ok so firstly lets assume that Jazz hasn't weaned by the time bub is born.
Can I tandem feed bub? My first answer to myself is no, because I haven't had bub so my body doesn't know that it needs to feed a newborn. Because the constitution of themilk changes over time, so the milk I'm making for Jazz (who will be at least 2.5 years old) would not be ok for a newborn. Is that right?

If Jazz has weaned, and then I relactate, will the milk be ok for a newborn? I have this thing in my head that if Jazz weans, and then I relactate, my body will go back to thinking I am feeding Jazz - a toddler - and not a newborn? Like resuming where we stopped kind of thing?

I need some really thorough resources on relactation and non-biological parents breastfeeding. I know the ABA has some I think, I haven't realy looked yet. Mums who adopt, and even dads (!) can breastfeed so I *KNOW* it's possible but I need to understand the biological process. How does my body make milk for a newborn if I haven't physically had a newborn? Will I need to take hormones? Is it possible to tandem feed if I haven't physically been pregnant or had a newborn?