Roughly speaking, 草鞋｛わらじ｝ is a shoe made of straws.

I'll show you the nuance:  
You say "to stand in someone's shoes" which is an idiom meaning "to see from another's point of view; to feel what another feels".
If you stand in your own shoes, and at the same if you stand in someone's shoes, what happens?
You can see from the point of both your view and someone's, right?

So, in Japan, if you wear two kinds of 草鞋 at the same time, you can be engaged in two trades.