Here is an image of the problem that I think is a bug. In the screenshot, I am using the Editor Extensions mod for convenience (but it can be reproduced quite easily in vanilla Space Age).
Steps to reproduce:
1. Place 5 thrusters horizontally (or any odd number).
2. Attach FUEL pipe to top connection on the left end.
3. Attach OXIDIZER pipe to top connection on the right end.
4. Send ship on a trip and watch as alternate thrusters are working (despite their neighbours having no supply).