I’ve always used 2 monitors when I work - it makes my life easier.
I’m on a mac: 10.3.6
Support for this type of functionality should be basic as few of us can afford professional 30" monitors.
I also need support for multiple monitors. I currently support 4 monitors with Display Fusion Pro 9.3 and a Sapphire ATI AMD Radeon HD 7870 graphics card with 2048MB of Video memory. My primary monitor is a ViewSonic VP2770 28-inch screen at 2560x1440 resolution, with 10-bit color depth and Adobe RGB color space. My other monitors are ViewSonic VX2450 24 inch screens at 1920x1080 resolution with 8 bit color depth. I profile my monitors every other week with an X-Rite ColorMunki Photo and calibrate my monitors and Epson Stylus Pro printers.
I am using a PC with Windows 10 Pro 64-bit, an Intel Core i7 4790K @ 4.00GHz processor, 32 GB RAM, a SAMSUNG 256 GB (SSD) scratch disk, a Samsung SSD 850 EVO 500GB (SSD) system drive, and 8 TB of local storage using 3 Western Digital 4TB Re Datacenter 7200 rpm SATA III hard drives in a RAID 5 configuration with Intel Rapid Storage. I use an HP StoreEver LTO-5 Ultrium 3000 tape drive for removable off-site storage and I run backups with NovaBackup Pro to the tape drive daily. I use a Synology DS1815+ Disk Station 8-Bay NAS with 6 Western Digital 4TB Re Datacenter 7200 rpm SATA III hard drives in a RAID 5 for 20TB near line storage.
I have three monitors; I keep one at lower resolution just so I can read things from software that doesn’t support 4K (menus, etc.) I’m on a custom-built PC.
also voted. Would like to have a full screen view on a 2nd screen whilst having all controls on the 1st screen (or vice versa of course 
See also my topic on this here from a few months ago when there was no voting system in the Forum.