The above wont work, as it relies on both tasks being complete.
Preds/succs arent conditional. If task 2 is linked from the completion of tasks 1 and 2, then both need to be complete before it will start. The only way I can think of is to put a note on the task, and manually check it when its going to start.
You may be able to use resourcing, by allocating the same resource to each task, setting a resource limit of 2, and linking from the earlier finishing task, then resource levelling, but I'm not a fan of levelling as it tends to do things you dont want. If however you dont have resourcing, it could well solve the problem- take a bit of thinking, but its a possible solution.