You can open as many Buried Treasure as you have Treasure Maps to get immediate rewards if you'd like. The 2 per day limit is only on the Nautical Crates which don't grant event rewards, only event items (Map Fragments) and a chance at Hypnocats, Doctor's Notes, Pizza Bikinis, and Care Packages.
Ok I might be slow on this or do not understand what was said but..why are rewards based on buried treasures? It should be based off the treasure maps. Buried treasures are available in the store..so...yeah I don't get it.