Hello and welcome to justanswer.com my name is XXXXX XXXXX I'm here to help you with your TV problem.
Is the DVD player also a VCR player since you mention both.
Easiest thing is turn off the DVD/VCR and if the dual screen stays then it's the TV.
If it's the TV look, please on the remote for a button probably called PIP or P&P (or it may have an icon of a TV with a line down the middle) and press it a few times to turn this feature off.
If no luck then what is the model number of your TV and I can help you further figure it out.