I'm curious if there are any arts that just don't work together. Where training both arts will actually hinder your progress more than help it. Now, I know if you train something like Brazilian Jiu-Jitsu and Muay Thai, that what you'll get is different techniques for different situations (BJJ when you're on the ground, MT when you're standing up). So even though they have different techniques, they can work well together. Similarly, I know people might have an issue training similar arts with slight differences between them, for example if you were to take Shotokan karate and Kukkiwon Taekwondo, you'll notice a big difference in the stances. But, if you're capable of doing a back stance and front stance, and if you're capable of doing a front kick and a roundhouse kick, then you should be capable of doing a Shotokan-style stance and a Kukkiwon-style stance. But are there any arts that are fundamentally so different from each other, that cover the same fight situations as each other, where training in both will make it harder to train in either of them?