How do you make 2 monitors act as one in 10.10.4

Do you mean having one app extent across both screen?

Use Spaces

OS X Yosemite: Work in multiple spaces

The Mac has supported multiple displays since system 4.5 (circa 1987).

If you choose "Extended Desktop", and the area from the second screen will be added to an edge of your existing screen, extending it in the direction you specify. Then place additional Windows on either screen, or even split across screens.

If you choose "Mirroring" both displays will change resolutions to a resolution both can support, and the displays will be showing the same thing.

If you are using Mountain Lion or earlier, this article describes the features available and how to use them:

OS X: How to use multiple displays with your Mac in Mountain Lion and earlier - Apple Support

Regardless of what Mac OS X version you are running, you should become familiar with those features, especially under the Extended Desktop and Mirroring sections. Later versions of Mac OS X add some more complicated options in addition to all those features.

The main complicating feature is taken directly from iOS, the land of tiny screens and one-at-a-time processing. It is called "Displays have Separate Spaces" and is only helpful if you want to have several different Applications assigned to several different displays. Otherwise it is a nuisance and needs to be turned OFF.

This is accomplished completely counter-intuitively in Mission Control:

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Maybe this will apply to Yosemite

http://www.imore.com/how-span-window-between-two-displays-mavericks

http://apple.stackexchange.com/questions/114449/macosx-maverick-10-9-how-do-i-st retch-a-window-across-both-of-my-dual-sc…

https://www.youtube.com/watch?v=k5K5GNQjdRE

Tutorial: How to span one wallpaper over two monitors on a Mac!