I have just recently achieved this successfully at our church. It took a lot of trial and error but the below information will get you up and running first time!
We have an iMac that is utilising a Matrox TripleHead2Go (display port to HDMI) and an ATEM TVS.
Our AV system diagram is at the bottom of this post: (or here -->
http://postimg.org/image/8xvhi5grl/)
First point is that the Matrox cannot feed directly into the ATEM (at least after months of research I've never heard of it being possible and I tried everything that I could think of without success).
Therefore you need a
scaler to conform the signal leaving the Matrox TH2G into a format that the TVS likes. I would highly recommend the Redbite Decimator HDMX (
http://www.bhphotovideo.com/c/product/1091826-REG/decimator_dd_hx_hdmi_sdi_cross_converter_with.html). We use this device and it's perfect, not to mention packed with extra features. If you're considering another scaler take a look at the ATEMuser 'Compatible Equipment' list (
http://atemuser.com/CompatibleEquipment) to see if the gear you have in mind might work. Note: We initially tried a Gefen Toolbox 1080 Scaler but found it only worked if sending 720p to the ATEM (and the ATEM would need to be set to 720p format in this case). Using the Gefen to scale to 1080i 50 would produce signal at the ATEM however all the colours would be incorrect (red channel missing?) and there's no fix no matter what settings are tweaked in the Gefen Toolbox. Decimator MDHX works at either 720p or 1080 50i so it's a perfect choice all round.
We have the Matrox TH2G sending signal out ports 1 and 3 (we don't use the middle port) and we use Propresenter with the Multiscreen Module ($200USD) to send two unique feeds (set up to 'tile' rather than one wide display); one into the ATEM with lyrics on a blue 'chroma keyable' BG which we key over live footage via the ATEMs upstream key (live on main projectors), and the other the same lyrics but on a black background which appears on stage monitors.
For some reason setting the Matrox up to send 2*1080 50i would produce no signal on our stage monitors, (just black), but the other port (port 3) would be fine through the Decimator and into the ATEM. We needed both to work.
I then dropped the Matrox down to be outputting 2*720 50Hz which produced the image successfully on the stage monitors (port 1 out of the TH2G). The other port (port 3 out of the TH2G) was then sending 720p 50 into the Decimator which was upscaling to 1080 50i which was the format to which the ATEM TVS is set (our cameras can only do 1080 50i....not 720p).
This configuration allows a single iMac operator to run lyrics to both sets of screens, but with different backgrounds. We wanted this functionality because we don't want live camera on the stage monitors but rather just lyrics, and also wanted to keep the ability to key in the ATEM to send lyrics over live footage to our main screens.
If you're trying to get a similar setup running I'm only too happy to chat through anything.
Just email me lance.a.clayton[at]gmail.com.