A unique selectively openable seal line made between two sheets of flexible thermoplastic material is disclosed, the seal line being exceptionally resistant to unintentional opening forces but being easy to open upon the application of a specific force. The seal line may be employed in various containers, including a two chamber container for the separate storage and selective mixing of two medical substances. The container includes two sheets forming the exterior of the container and an inner, diaphgram sheet between the outer sheets. The selectively openable seal line is disposed between one of the outer sheets and the diaphragm sheet. Preferably a permanent line of securement is also included, between the other exterior sheet and the inner diaphragm sheet, extending substantially parallel to and coextensive with the openable seal line. Such a container provides a relatively inexpensive and reliable means for separately storing two medical substances in a closed system under sterile conditions, while still providing for quick and easy selective mixing of the substances when desired. Finally, the present invention is also directed to a sealing die for making the unique seal line and to a method for manufacturing the container.