Yes, this was a limitation of the software as originally designed. Whenever we add the complication of a steering servo we must steal it from another existing RC output and it was easier (mostly from a user-interface point of view) to hardcode it to RC Output 2, which precluded the use of dual ESCs in halftrack mode. I also did not expect many people to use ESCs anyway since serial controllers are superior in every way unless you are driving brushless motors, but I have been surprised at how many people still choose them.
I have released a firmware update that will fix this (you will have to update OP Config and TCB firmware both - version 0.92.20)
Here is the new behavior - if you want to use dual ESCs for halftrack mode, the steering servo will be assigned to RC Output 4, which means you can no longer use that output for the barrel elevation. In your case it doesn't matter. For other people who might still have a turret on their halftrack, they will just have to use the onboard driver for barrel elevation (or they can map the barrel elevation stick to yet another RC output if they have one open, which they might depending on the sound card used).
If you don't need dual ESCs in halftrack mode (for example you use a serial motor controller instead of ESCs), or if your halftrack only has a single rear drive, then the steering servo will be assigned to RC Output 2 as usual.
Either way, OP Config will tell you where to plug in your steering servo when you make your selections, so hopefully that makes it easy.
Please let me know if you have any problems.