Conservation auctions are designed to allocate payments for environmental services to voluntary farmers. The auctioneer may announce either the maximum number of contracts (target-constrained auction) or, more commonly, the total budget available (budget-constrained auction). Building on previous work that compared these two formats, we introduce a new double-constraint auction, where both constraints—set at the same levels as in the benchmarks—are simultaneously disclosed to participants. Using the same experimental methodology, we assess performance consistent with a generic policy objective: maximizing environmental benefits while minimizing expenditures. On average, the double-constraint auction outperforms both target- and budget-constrained formats.