Yes it is possible do it using a feature code.
first to display the codes logon and type
dis-wabe:gen,,,,signon&signoff;
Here you will see the codes for same(different for each customer)
The type
reg-persi:si;
here you will see another code which is required.
At phone A dial signoff code then si code and A extn number(phone A then signed off)
At phone B dial signoff code then si code and B extn number(phone B then signed off)
At phone A dial signon code then si code and B extn number (B now woking at A position)
At phone B dial signon code then si code and A extn number (A now woking at B position)
Regards