MTS is defined by your MTS_ parameters in the init.ora file. Take out those parameters and MTS is disabled. I have read recently where the MTS_ parameters can be changed on the fly, but have not tested.

The easiest way for you to switch is to use the (SERVER=DEDICATED) information in your TNSNAMES.ORA file. Using this switch will guarantee that each connection to that alias will use a dedicated server. This way, you can change your init.ora when it is convienient and bounce the database. For example:
Code:
TEST=
   (DESCRIPTION=
      (ADDRESS=(PROTOCOL=TCP)(HOST=dev1)(PORT=1521))
      (CONNECT_DATA=(SID=test))
   )
becomes:
Code:
TEST=
   (DESCRIPTION=
      (ADDRESS=(PROTOCOL=TCP)(HOST=dev1)(PORT=1521))
      (CONNECT_DATA=(SID=test)(SERVER=DEDICATED))
   )