Apple's iMac with Thunderbolt lets one iMac use another as a second display, while the second iMac remains a fully-functional system in the background. I tailor each iMac for the tasks they run -- desktop software or simulation. My primary iMac is a 5K retina 27" i5 and 16GB. The secondary iMac is an older non-retina but maxed-out with a quad core I7 and 32GB. Both have SSD main drives, with a 4TB OWC Thunderbolt RAID array.
I can boot the secondary iMac in either MacOS or Windows, and then use its screen directly to set up a simulation before swapping the screen to my primary iMac (cmd-F2) as a second monitor. To check up on simulation status, I VNC into the secondary machine using the OS X ScreenSharing utility. This lets me enjoy the desktop advantages of the retina display and a second monitor for basic tasks like CAD and presentations, without being dragged down by CPU-intensive simulations,