Yes, you can put 2 Forever stamps on an envelope. Forever stamps are valid postage and can be combined to meet the required postage amount for any mail class. If one Forever stamp provides insufficient postage for your intended mail service, adding a second stamp will increase the total postage value. For example, two Forever stamps equal the cost of two first-class letters, so they could be used on a single piece of mail that requires that amount of postage, such as a heavier letter or a package sent via certain services. The USPS simply requires that the total postage affixed meets or exceeds what's needed for your mail piece and service class. You can also combine Forever stamps with other denominations of stamps to reach the correct amount. However, for standard single-piece first-class mail within the United States, one Forever stamp is typically sufficient, so using two would result in overpaying. Forever stamps don't expire and remain valid for first-class postage regardless of future price increases, making them useful for building up a supply. The key rule is that any combination of valid stamps totaling at least the required postage will work—the USPS doesn't restrict how many individual stamps you use on an envelope as long as the total postage is correct.