The USB protocol doesn't allow for direct device-device connections, for this to work you'd need one of the devices to act as host. You can however use a USB OTG cable to do this as the USB OTG cable tells the device it is plugged into that it needs to switch to host mode although this does require the device it is plugged into to support USB OTG, which the Nexus does. So you'd need to plug in a micro USB OTG adaptor into the Nexus 7 and a regular USB-microUSB cable into that to connect the devices, the next issue here is that the Nexus 7 doesn't support mounting storage devices out of the box, but it does apparently support MTP devices. So if your phone presents itself as a mass storage device you'll also need to root the Nexus 7 and install Stickmount to mount the phone's storage. It may also possible to do this the other way around with the USB OTG adaptor plugged into the phone, but that depends on the phone's capabilities.
Sent from my Nexus 7 using Android Central Forums