Certainly,
but why do you have to change anything?
make the task, give it duration 1 day (or 8h if you prefer), then in the resource sheet make the two resources, A and B.
Then back to the Gantt chart, select the task, then resource, assign resources (button), select each one and assign to the task.
It's done. They each have 8 hours of work.
Assign A and B at the same time. If you assign A and then later assign B, that may change the duration, depending on the task type and effort driven settings in task information, advanced. The default is fixed units and effort driven off (what are yours?), and in that case the duration will not change, but if someone has changed them, the duration may change. If it does, just type it back in.
And if you want to see who does what when, go to the task usage view or resource usage view.