Indeed, two prepositions aren't frequently together, but this is a pretty common set expression. It talks about something/someone that interacts or interferes with something/someone else.
- No gané el concurso porque hubo dinero de por medio.
I didn't win the contest because there was money involved. (Someone else paid to win.)
- Quería insultar a tu esposo, pero estás tú de por medio.
I wanted to insult your husband, but you're in the middle of us. (I don't want to make things bad for you in your marriage, despite the fact that I hate your husband.)
- No tiene sentido hablar contigo, si no hay de por medio una verdadera intención de comunicación.
It's useless talking to you when there is no real intention to communicate between us.
- Hay muchos intereses de por medio en este negocio.
There are plenty of individual interests involved in this business.