Most communities are matchmaking sites.
You're trying to match someone with a problem to someone who knows the solution.
There's a critical mass about these things. You need the abundance of one side to attract the other. And that's the secret here. You should target and appeal to one side. One side will attract the other.
If we have a question about programming, we go to a community where developers gather. We tend to recruit developers and programmers from these communities too. The paid work opportunities attracts more developers and programmers.
StackExchange also thrives on this model. ModeyMayhem began by appealing to models. This attracted the photographers. The photographers attracted more models etc…
Don't try to appeal to both sides. Pick one and design the concept to appeal to them.