Container unit comprising two containers

A container unit for enabling flow-communication with a second container unit while engaging the second container unit to form an extended single compartment container by combining the volume of both containers and expanding the available mixing space, comprising:

• a) a neck portion part comprises:
• i) a cylindrically shaped portion at an upper end of the neck portion part, including a sealable opening, wherein said neck portion part configured to be inserted into an opening of the second container unit;
• ii) a ring-like seal located at the upper end of the neck portion part, the ring-like seal is a disk-like sealing flange formed of flexible material and sized wider than said opening of the second container unit to seal said opening of the second container unit when inserted therein, wherein said ring-like seal portion is configured to be inserted into said opening of the second container unit, wherein the ring-like seal forms a fluid-tight seal with an inside wall surface of said opening of the second container unit when the top end of the container unit is inserted into said opening of the second container unit, whereby said container unit is in flow-communication with the second container unit through the top open end of the container unit and said opening of the second container unit;
• and
• iii) a shoulder spaced apart from said ring-like seal, wherein said shoulder sized and configured to sit on and engage to an open edge of said opening of said second container unit, so that while said cylindrically shaped portion of said neck portion part is inserted into said opening of said second container, said shoulder forms a removable abutting relation with the open edge of the second container;
• b) a container body part with an opening at a top end;
• wherein the container body part and the neck portion part are being sealingly coupled to form said container unit.