Does anyone know of a solution that allows 2 monitors to be one continues monitor on the Mac Pro (Late 2013) like my older Mac Pro (2006)?

Be sure to DISABLE "displays have separate spaces", here:

Sure, I'll try. go to the big Apple in the corner, then system preferences, then Display, then Arrangement, then follow the onscreen instructions. Looks like you have 1 DisplayPort, one MiniDisplayPort and an HDMI, should you wish to use that. Your 2 monitors also support MST (multi Stream Transport) with displayPort 1.2 as there is a DisplayPort in and a DisplayPort out. Alas, I don't know if your Late 2013 Mac Pro supports MST....apparently it does, but not daisy-chaining 2 monitors via DisplayPort--thunderbolt only . Huh...didn't know that. Oh, and make sure Mirror displays checkbox isn't selected. You should be able to drag one display to the desired position to make it the primary display, and you'll get a red border around the display as it's being moved. If you don't like that, you can drag the other display over to make it the primary display.

here's how to do split screen on a Mac in OS Catalina, El Capitan, High Sierra and Mojave ....link....

https://support.apple.com/en-us/HT204948

anyways, hope this helps you

john b

Make sure the displays are not on the same Thunderbolt bus.

Use multiple displays with your Mac Pro (Late 2013) - Apple Support