Tidus needs to be at least Lv16 and needs to have learned Anti-Venom 2. You can learn AV2 in two ways; either techcopy all his Key Techniques in order from left to right, or use a Tech Find to open the slot and techcopy it prematurely. Then you need to win JS2 from a Tournament.

Linna and Brother start with Anti-Venom 2, but you can also win it as a Second Place prize in a Tournament.