Being in two places at one time?

How do I manage this?  Unfortunately, I don't have any family that lives close by and my husband is gone 7am-7pm with work and traveling into the city so it is me and the kids together 24/7.  I really don't mind until one of them has something at school that is the same time as the other one.  A perfect example was that my daughter had a skating show at 10:30am but I needed to pick my son up from school at 11am and just couldn't logistically do it so I didn't make it to the skating show.  L pointed out that she was the only one that didn't have family there....insert major mommy guilt.  I was asked to come and read in R's class but couldn't do it because it was one of the days that L didn't have school and I couldn't bring her with me.  
How do working moms do this?  How do other moms do this?  Anyone have good suggestions about how to handle this?  I try to balance them off and switch off who gets me for the holidays and sometimes the times work out but most of the time they don't.  Sometimes C can work from home so that I can go to a show while he picks the other child up from school but this doesn't happen too often.  It would be so great if I could clone myself; I'm sure that I am not the only mom that feels this way;)