Help with summoning friends

So I was trying to play with two friends earlier and we were unable to summon more than one of us at a time. We've played together frequently before so there was no issue of passwords being off etc. My friend was able to summon either one of us, but never both at the same time. When he would summon one of us, the other persons summon sign would disappear. Has anyone else had this problem before?

I don't know if it matters, but he's a level 60 summoning a lvl 73 and 99 in Anor Londo. Like I said, he was able to summon both of us individually, but not at the same time.