EC5 2 to 1 Series Adapter from BenchCraft - BCT5061-029
Features:

Allows you to connect two batteries to one ESC in series.
This will increase the the voltage available to the ESC and motor but will not increase the capacity. For example, you can connect two 3000 mAh 4 cell batteries to an ESC with this adapter and the ESC will see a single 8 cell 3000 mAh battery.

Specifications:

Length: 40mm \/ 4cm
Connector Type: Two male EC5 Connectors and one female EC5 Connector