Maybe, but there are big compatibility limits. Yeah, this one's common.
DDM only works with supported Dell monitors. It explicitly does not support Dell monitor models from before 2013 or the Dell 'D' series. If your old monitor falls into those categories, DDM won't control it at all.
If the old monitor is a supported model, you can control both. When multiple Dell monitors are connected, you can select a target monitor from a dropdown in the DDM Quick Settings dialog. A brief icon flash on the selected monitor confirms which one you're controlling.
If that didn't work: For basic brightness/contrast control across multiple supported monitors, enable 'Enable display matrix control' on the Basic tab. This applies preset adjustments to all monitors. For advanced features like Easy Arrange across monitors, enable 'Span multiple monitors' in the Easy Arrange settings.
Heads up: Using a mix of monitor models or resolutions is fine for DDM's window restore feature. It can recognize a reconnected monitor and restore application window positions accordingly, even if the models differ.