Your diagram showed the two coming together at the house - I had expected that would be at your distribution panel, where you would be able to access all the cables.
In this case, that first option is not available to you. You have little choice but to have one emonTx in the garage, and one emonTx in the house. Plus an emonBase - also in the house, I expect - to collect the data.
You will need a programmer, and you will need to download the 3-phase sketch, customise it according to your needs, and upload it into each emonTx.
And you need the emonBase. You will also need to set up the Arduino IDE on your computer to upload the sketch.