I posted several times while I was pregnant with DS2 and still bf DS1 about my milk not changing to colostrum at any point.
Just to let you know, as far as I know, my milk never changed during my pregnancy and I am tandem feeding fine! My breasts filled up (massively) about 3 days after DS2 was born as normal, and then settled down again a couple of days after that.
One thing I wanted to note was that DS2 never lost any weight after birth. Don't know if that's normal when tandem feeding or just one of those things. He's also sleeping much better than DS1 ever did at only 11 days.
The mw said my milk must be really good, and while I'd like to believe it, I'm a bit about that!
I think all babies are different so it's tough to say if what is happening is down to tandem feeding. I did find my supply regulated more quickly but I've also heard this happens with second babies regardless of whether you are tandemming or not- who knows!
I'm sure your milk is fab, breastmilk is great stuff Your MW was probably just trying to be encouraging and it's a nice thing to say if a little odd.