The HD5450 supports at least 3 displays.
How many Video Outputs does your GPU card have? DVI, HDMI, DP, or VGA Video Outputs.
You can always use an active adapter to convert one of the video outputs on your GPU card to make an second Monitor to work in case you don't have 2 of the same type of video outputs.
I have three outputs in my graphics card (DVI-I, HDMI and VGA).
And my monitor has it (DVI-D and VGA).
I am now working on (VGA from my graphics card to VGA in monitor).
How can I run the new screen alongside the current screen?
Connect the second monitor/display to the other 2 remaining video outputs that corresponds to the type of Monitor you have (DVI-d or VGA or HDMI) with the correct monitor cable.
Please post both Monitors Make & Model and your GPU Make & Model and how you are planning on connecting the second monitor and which type of cable you are planning on using.