Yeah Laura is right, it could be something simple going around, like an upset stomach. It could also be a coincidence like furballs or just random. My cat Jack is sick quite often, furballs and bits of grass mainly. I think some cats have sensitive stomachs and need a special diet. Such as dry sensitive food to help regulate everything.
As long as they are healthy, have healthy eyes (no third eyelid showing) drink plenty and have a healthy appetite I don't think it'd be too much to worry about. If they start to get mushy watery stools though perhaps they could do with a check over. I hope that helps, our animals are sometimes a mystery to us I know