How can I merge a partitioned internal hard drive? (Macbook pro 15" i7, with Yosemite 10.10.3)

I split my internal 500gb flash disk storage into 2x 250gb partitions when attempting to split an external hard drive. Only one of the partitions has any data on it, but it is getting close to filling up and would like access to the other unused 250gb partition. How can I merge the two partitions into one?

Specs:

Macbook Pro 15" Retina Late 2013

2.3 GHz Intel Core i7

16 GB 1600 MHz DDR3

Intel Iris Pro/ NVIDIA GeForce GT 750M

USED PARTITION: Journaled HFS+, writable, unlocked.

UNUSED PARTITION: Same but none of the 250gb has been used. Partition Map Type: GPT (GUID Partition Table)

Thanks

C.H.