I have a FS CT6 tx/rx and I'm having issues. Basically I want to use this for robotics, and not to fly any rc plane/heli. When I move 1 joystick upward, both servos should go forward and when I move it downward both servos should go back. However, when I tried this, both motors started rotating without me giving any input to the tx.

How can I accomplish this? Do I need to use Mixing and the ACRO setting?

yup

you can use the potentiometer for that

Please elaborate.

Are you using a continuous rotation Servo or a standard 120 degree servo (60 degree of rotation in each side)?
Did the servo arms ever stopped?
A short video should also help us in ascertaining the issue.

The easiest way to accomplish it is by using a Servo Y-Lead, Provided that you want both servos to move in the same direction:
http://stores.ebay.in/Indian-Hobby-Shop/_i.html?_nkw=y+lead&submit=Search&_sid=1125855133

I have connected 1 servo to Channel 1 and the other to Channel 2. In the configuration software, I set the left joystick to make Channel 1&2 to go forward with max speed when the joystick is pushed upward, and vice versa for when it is pressed downwards. However, at the neutral position, 1 servo is rotates at normal speed and the other is rotating at a slow speed. I tried to make it stop by moving the trims but it hasn't worked. I found this very odd because in the configuration software, it showed that at neutral position neither servo was being turned on. The servos are continuous rotation.

Wouldn't a Y-lead drastically reduce the voltage received by each servo? Also if I used a Y-lead, how would the robot turn right or left? When turning, 1 servo needs to rotate clockwise and the other anticlockwise, how can this be accomplished using a Y-lead?

I'll try uploading a video later, don't have access to a decent recorder right now.