I've done SL since P3/4 with my oldest, who is now 8. We're doing Core B now, because I deliberately slowed her down by adding a ton of stuff to Core B, since I've heard from others that you don't really want to start Core D too soon.
For combining, I find that the early cores are light enough that it makes sense to keep each child at their own level. For instance, I have a 6-year-old (was 5 at the start of the school year, so she's kindergarten) and an older 4-year-old (was 4 at the start of the school year), so they're a year apart. This year, I had the older of the two doing P4/5 and the younger P3/4. I would just read one P3/4 book first, both listening, then I'd do the schedule P4/5 readings and the younger one was welcome to listen as she wished or wander off to play.
I just started combining the two of them, though. I'm doing my own thing for a year, a sort of geography tour of the world, but using a lot of books from P4/5 again, which the 4-year-old is now ready for. When I finish, probably sometime this fall/winter, I'll combine the two of them together in Core A. So combining can work, but I just don't think it's worth it until you get to at least about Core A when the reading starts to pick up more.
Another reason I like to wait a bit to combine them is, well, I'm not actually combining my oldest two. My oldest is 8, then the others are 6, 4, 2, and a new baby. I'm combining the 6-year-old and the 4-year-old and keeping the oldest on her own core. The reason is that she's huge on being read to and has a ton of patience for being read to, plus she's not super sensitive and is fairly mature. She needs to be doing a core that is on her level. My 6-year-old likes being read to but doesn't have the infinite patience for it that her older sister had at her age, plus she's very sensitive. If I tried to combine her with the 8-year-old, I'd have to "hold back" the 8-year-old from where she should be. But combining the 6-year-old with her 4-year-old sister works much better. The two of them pair together very well and have similar imaginations/playtime styles, whereas the older has her own style and fights with the 6-year-old a lot.
All this to say, if you have more than two kids, waiting until you see their learning personalities emerge lets you make a better decision about who to combine with whom and when and into which core.