My twins are 2 3/4... they've been weaned since just before they turned 2.
I only nursed them together at night - so I could get more sleep. At first when we were home from the NICU, I'd wake them both every 2 hours... then every 3 hours.... then after they were better able to self-regulate, whenever one woke I would wake the other one and nurse him/her too. Again, this was just in the middle of the night.
But during the day I always tried very hard to pay attention to when each of them wanted to nurse as opposed to putting them on a schedule, and I also liked the separate mama time that nursing separately gave us. Granted, I did spend about 6 months straight on my sofa LOL but it was worth it. It probably also depends on the children... dd is a quiet nurser, who likes no distractions. Ds is an active nurser, who bobs on and off, looks around.... he was just plain annoying to poor dd who was trying to zone out and enjoy the quiet cuddling!
As they got older, whenever they wanted to nurse (whether together or separately) was fine with me, I guess it was about 75% of the time separately and 25% of the time it happened to be together. But we had a couple of simple ground rules that made life easier, such as no pestering each other (they actually learned this quickly and did really well with this - again, this was for dd's benefit), and if your sibling is currently nursing no you cannot switch sides right.this.instant!
Oh, edited to add... starting fairly early (6-9 months-ish?) I would make a point of cuddling with one while the other was nursing. This circumvented the whole jealous nursing thing to a large extent. Meaning, if dd wanted to nurse, I'd nurse her while cuddled with ds or tickling his feet (his favorite thing!) or something. When they were older, more toddler age, I'd read the other one a favorite book or whatever. I tried really hard to avoid the gang attack mode LOL which we definitely got into and was really hard (both wanting the same breast, etc.!)... this way they got to nurse whenever they wanted but they didn't think it was their only path to cuddling from mama either.