This is possible:
1) Start geth in Terminal first:
> geth --mine
2) Launch Mist, but also from Terminal so you can pass commands to make it connect to the IPC created by geth. On my computer, it looks something like this:
> /Applications/Mist.app/Contents/MacOS/Mist --rpc ~/Library/Ethereum/geth.ipc
Note: you need to change the paths above to correspond with where those files are on your computer and where you're calling them from, as well as the network.
The geth.ipc file is only created/shows up after you have started geth.
For example, another example of this with testnet is:
> geth --testnet --mine
Terminal 2 (from home folder)
> cd /Applications
> Mist.app/Contents/MacOS/Mist --rpc ~/Library/Ethereum/testnet/geth.ipc