Hi! Using an @lternate account to benefit a main account is not allowed. It is during the act of plundering that one account benefits. This means that when you plunder your alt or your alt is plundering you, there is always one account that benefits during that process.

In the case of Arena 3 v 3, it really sits on a grey area. Since, it is a feature where both accounts benefit and not just one, during the "action", it technically falls within the rules.