First, a disclaimer; I have a 7854 - not a 7904 - but they seem to have the same middle section and controls.
Set up 2 time base units as you need. Then you need to direct the triggering controls to the appropriate vertical plugins using the trigger source push buttons in that middle area; A is the left horizontal plugin, B is the right. Select either left or right vertical plugin as needed. Then use the right bank of illuminated buttons to chop or alternate the sweeps. Generally ALT works better for higher frequency sweeps while CHOP is better for slower stuff. I¡¯m sure there are plenty of exceptions.
-d
Doug Bercich