Basically they can! IIRC in the novel side stories, as DNI mentioned, it's pretty late in the SS, both of them seems to be capable of pregnancy, but just needs to get tested which body is more capable and more compatible of doing so.
Chi young actually wanted to carry the babies instead of heesung, because he felt like heesung is too young and too small to carry. But heesung also wanted to carry the babies since he's obsessed with "family". So they decided to get tested and heesung is the most compatible. Heesung was jumping for joy and chi young was sad for a bit but ultimately very happy for heesung since he knows what this means for him.
I forgot if chi young's constitution made him incapable of pregnancy or if was simply less compatible.
Mc COULD carry kids but it was with the help of sum family drug and it was still VERY hard on him because he was an alpha AND a man tye male physiology doesn't account for a baby being able to live in it or have the room for it like a women's does(the drug helped his body think it was an omega and made like a temporary womb or smt i thought it was weird tbh), in hessungs case he resembles more of a women structurally and was more fit for the role because (honestly idk can't remember just wanted to sound smart but kinda like what I said lol)

Can y'all tell me if they eventually get pups?