As Stuff has pointed out, five guys doing one day a week is not the same as one guy doing five days. You'll have five guys who say that week they can only do Monday, for a start. And anyway, why go for the hassle of an 80% commitment with your main employer and 20% with another in the first place?
Most airlines won't let you onto less than 100% commitment until you've served a couple of years,
AFAIK.