Q: Yosemite dual external monitors not recognized?

I have the same issue.

Here's how to prevent it:

Don't download buggy beta OS.

I;ve spent hours rtying to resolve this.  It requires an update from Apple.

So we are stuck with one display until then.

In the same boat, responding hoping to bump attention here.

Just bought a StarTech Thunderbolt dock to run a dual monitor setup off of my MacBook Air. I can get either monitor to work correctly on their own and can run one external monitor with the laptop monitor functioning, but it won't recognize two external monitors.

I have two ViewSonic VX2703MH-LED monitors and have tried using the StarTech onboard HDMI, Apple Thunderbolt to HDMI adapters, StarTech Thunderbolt to HDMI adapters, Apple Thunderbolt to VGA adapters, and tarTech Thunderbolt to VGA adapters. No problem getting one monitor to work in any configuration, but it won't recognize the second monitor in any configuration.

Exactly the same issue.  My Eizo Coloredge was working fine after installation until (just now) I dared to do a restart...

I have tried various different displays and connections over the past 20 minutes and have restarted my mac (late 2013 MBP 15' Retina) and connected / disconnected multiple times.  Neither my Coloredge via Thunderbolt or HDMI (no signal detected) is recognised, nor a Samsung LCD via HDMI.

SO frustrating that something this basic should now be 'broken' when it was fine under Mavericks (and earlier OS).  Come on Apple, let's get a fix.  Plenty of people rely on external monitors.

Sigh.

Identical issue.  Upgraded Macbook Pro (Mid 2012) from Mavericks to Yosemity and the OS no longer recognizes my external USB monitor.  Thuderbolt to DVI works fine.   USB to DVI or USB to HDMI don't work.

Now I have to figure out how to back-out of the upgrade and go back to Mavericks since 2 monitors is a must-have for me. 

Well - I solved my issue.   It's unlikely my solution will help many others since it's pretty specific to my config but I'll post just in case.

My issue, like others, was that upon upgrading to Yosemite one of my external monitors would not be recognized.  It just "wasn't there".

After switching monitors and connection types (hdmi - dvi) I was able to isolate it to whatever was plugged in to USB-(dvi/hdmi) adapter just wasn't working.  My adapter is a StarTech - DisplayLink USB 3.0 Dual Head Graphics unit.  I contacted StarTech support to see if they had a solution but they were unresponsive.  I searched drivers for DisplayLink 3.0 on their download site and found a Beta Mac version for Yosemite.  Loaded/installed it and my second (USB) monitor now works.  There are a few minor issues but it's 99% functional and the things that are off are just small visual items that don't affect usability.

I found this thread in a search.  Is anyone having success with two monitors connected to Yosemite?  If so, can you tell me the hardware you have and I will get that.

I will try again to see if anyone is having success with two monitors in Yosemite 10.1.1.  If so, can you tell me what hardware to buy?

Same issue - but I fixed it! I had the same issues with my MBP while running Mavericks. I tried updated to Yosemite (my co-worker runs Yosemite and has no monitor issues) to no avail. It still did not want to acknowledge any external monitors. Finally I did away with the HDMI connections altogether. I bought to DVI to Thunderbolt cords (Cable Matters, Gold Plated) from Amazon and BOOM! Fixed. Both monitors fire right up every time I plug it up.

I know its not a real fix...its still an issue that Apple needs to address, but it seems to be a great way to get around it until they do. Ive been using these cables for a couple of weeks now and still rocking. I hope this helps someone!

A little history for other to compare to.

MacBook Pro (Retina, 15-inch, Refurbished Late 2013)

Scenario 1: Mavericks 10.9.5

                    Monitor 1: HDMI to HDMI

                    Monitor 2: HDMI to HDMI via Mini to Thunderbolt Adapter

Scenario 2: Mavericks 10.9.5

                    Monitor 1: HDMI to Thunderbolt

                    Monitor 2: HDMI to Thunderbolt

Scenario 3: Yosemite Update

                    Monitor 1: HDMI to Thunderbolt

                    Monitor 2: HDMI to Thunderbolt

Scenario 4: Yosemite Update

                     Monitor 1: DVI to Thunderbolt

                     Monitor 2: DVI to Thunderbolt

Someone missed a day at work.  I upgraded to Yosemite and can't see/use my expensive second monitory.  This needs a fix fast!

Did you ever find out a fix to this? It's surprising but it's over a year later and I'm having this problem. Two Dell 27" 4k monitors. I can get one recognized at a time but it never recognizes the 2nd monitor. Either works fine on their own.

Thanks for the help.

nvm. actually just updated to el capitan and solved it.

I'm using a Mac Pro (coffee can  SN: F5KMD195F9VN) with dual monitors (not sure where the serial numbers are located, but they are standard 27in apple displays).  Everything was working great until I upgraded to El Capitan 10.11. 

Now only 1 monitor will work...  It does not detect the second monitor in the monitor's area of the preferences panel, however sound on the second monitor does seem to work with 2nd monitor plugged in...  If I unplug the first monitor and reboot, the second monitor will be auto detected as the primary, but will not detect the second monitor unless I reboot.  Once I do this the second monitor appears to be made the primary display and it will no longer detect the first monitor...

Help!!!  Thank you.

I'm curious if you resolved your issue. I have the exact problem you detailed. Strangely, I have two identical machines that are set up the same, but only one has the problem.