If you assign the resources one at a time the second resource 'assigned' the default is for effort driven so an 8 hour task becomes 4 hours in duration but the same Effort.  When you are assigning resources if you highlight both resources at the same it will remain as an 8 hour duration but the effort will be 16 hours...... This is the same default as MS Project.  You can go into the Task Advanced dialog and also change the task to "Fixed Duration"