I have one gridseed 5 chip miner and one gridseed g-blade. Can I run both of these miners off the same raspberry pi running minera?
yes you can
Do I need to use a powered usb hub? I tried a non powered hub and I got some wonky hashrates. Is that the cause?
Yes you need a good powered hub, I mean neither the cheap one want work.
This is very good if you don't need lot of ports:
http://www.amazon.com/D-Link-Hi-Speed-7-Port-Powered-DUB-H7/dp/B00008VFAF