In Pathfinder, can you gain more than one martial discipline at the same time by just taking the Martial Training feat twice?
Not from Martial Training.
You cannot gain more than one martial discipline by taking the Martial Training 1 feat twice, because you can only take the Martial Training 1 feat once.
First, note an example feat, Skill Focus, which contains this text:
Special: You can gain this feat multiple times. Its effects do not stack. Each time you take the feat, it applies to a new skill.
The Martial Training 1 feat does not say you can take it multiple times, therefore you can only take that feat once. Martial Training 2 and the rest of the chain grant you additional maneuvers from the chosen discipline, not access to a new discipline.