Do you mean physical seat allocations on the aircraft, such as seat 1A?

If so, are you looking at a flight on which there is only one seat available in the cabin to allocate? Otherwise, by definition there are already at least two seats available so no alert can be set for that condition.

If there is only one seat available in the cabin, and you want to an alert for whenever a second seat becomes available, then one way is simply to set one seat alert for every unavailable seat in the cabin. You'll get alerted the first time that any of them becomes available. If the originally-available seat has not become unavailable in the meantime, the effect of the notification is that there are two seats available because a second seat has come free. This is not perfect because the originally-available seat may have become unavailable in the interim, but this might go some way towards doing what you want.