I suppose that by vertical bar you mean the rectangular box along the object's lifeline(dashed line). The rectangle is called the focus of control and indicates that the object is actively doing something at a given point in time of its lifetime. (For example, it could be sending messages to another object or it is in the middle of executing something). It doesn't correspond to the method call but depicts if the object is active or not at a given point in time.
If I understand you correctly, you want to show two distinct focuses of control on Object B. That is you want to depict along the lifeline of object B, a rectangle followed by a dashed line followed by another rectangle - all along the same vertical axis.
I just tried this on Posiedon but it won't allow this representation of mutliple focuses of control for one object. However, the sequence diagram example on the below mentioned link shows multiple focuses of control on a single object. I guess you are trying to depict something like this right? http://www.gentleware.com/products/documentation/poseidon_users_guide/x3564.html
Sorry couldn't be of much help here. I have had my frustrating moments with Poseidon and know what it feels like. You may want to pose your question to Poseidon support or Poseidon users forum (think there is one on Gentleware's website). Good luck!
[ April 22, 2004: Message edited by: Sridhar Raman ]