Right click desktop and select display settings. Scroll down to multiple displays and use the drop down to select Extend these displays. Hopefully that is what you are after.
If that's the case, and it's duplicated instead, it makes sense. But, when you extend you have to do one of two things: have 2 separate EQ installs so they can exist side by side(and I would recommend turning on switch on mousehover) or use a program like ISBoxer to make and use virtual separate eqclient.ini files.
If I can ever get my setup in 1 picture lol, you'd see that's how I have my twins only one 1 screen. Made one set up as I wanted, duplicated the EQ folder and installed a second launcher, renaming both to their charnames, and moved them to opposite halves of the same screen. To do this best I went into each eqclient.ini file and set the offsets and sizes so they were half and half width and full height. Screen realestate is a little at a premium doing it that way rather than each on its own, but it's the same thing as extending on a second, and 5 screens is all I have space for, this split screen mounted in the air as it is....I managed to do similar with their respective keyboards using mini laptop risers so I'm not resting on one to use another.