using second monitor to show diffrent information

I am working from home - though my pc - I have set up my second monitor - which is a dell - my mac is a desk to

I need to have the dell work excel and my mac for work email - it need to work off both - when i try it just has same thing on both - like my email on both when i go excel it does the same on both - how do i have two diffrent things on each monitor