@G Money XL Questions like this can usually be answered by checking the User Guide.  Here is a direct link, but you can find this type of stuff for almost any Dell product by going to support.dell.com, searching the device model name, and clicking the Documentation tab.  The feature you're looking for is called Picture-by-picture, or PBP.  I don't see it mentioned in the documentation of this display, which isn't really surprising because the U2721 is a QHD display (2560x1440), which means splitting it would either give you two tiles of 1280x1440 or two tiles of 2560x720.  Neither of those would be very usable.  PBP is more commonly found on "ultrawide" displays that have resolutions like 3440x1440, or even 5120x1440 as found on the Dell U4919DW, which is essentially two QHD displays fused together, so you can either have a "dual QHD as a single panel" setup for a single PC or use it as two separate QHD displays for two separate systems.

Dell Display Manager (not Dell Device Manager) is meant to allow a single system to carve up a display into multiple regions.  It is not meant to allow multiple systems to share a single display.  So even if you had a display that offered PBP and you used it to display both laptops simultaneously, each system that had Dell Display Manager would only be able to further carve up its own "tile".