You can change the behavior of Alt + Tab with an option in compizconfig-settings-manager that lets you group the applications together from the active viewport.
To install CCSM, run the following command in a terminal:
sudo apt-get install compizconfig-settings-manager
To enable the setting click on Ubuntu Unity plugin and select Switcher tab and tick Bias alt-tab sorting to prefer windows on the current viewport.
This will ensure that the applications from the same viewport will get grouped together in the window switcher. However this will not remove everything that isn't on your viewport. It just groups them together making it easier to keep them apart.
If you want to completely remove the other windows from your not active viewport from your alt tab menu you'll need to a different switcher like the static application switcher. You can activate the static switcher by doing the following:
click the Ubuntu Unity Plugin and go to the switcher tab. In this tab you'll need to disable the hotkey to start the switcher, the hotkey to flip through the windows in the switcher and the one to switch through the windows backwards.
Enable the Static Application Switcher by checking Window Management ▸ Static Application Switcher ▸ Enable Static Application Switcher
You should only switch between windows on your active viewport like you did on your old install.
If you do not get the static application switcher option you can try running the following command:
sudo apt-get install compiz-plugins-extra
I do have a little warning though:
What are some of the issues with ccsm and why would I want to avoid it?
If this didn't work you can comment and I'll post another method.