I bought a Eyc T plus front light for my bicycle. I then realised that the light is missing a cable outlet for the rear light (a in the picture). How do I make the rear light work. Do I simply try to squeeze in the cables of both lights into the hub dynamo?
If there's a light sensor or light switch, you have a problem, as that sensor or switch won't control the rear light then. The best solution could be to return that light unused and buy some other light instead.
I suspect the "T" part (tagfahrlicht, daytime running light) is the problem. This light is supposed to be on at all times, even during the day. This model appears to not have a switch even for the front light!
So yes, you can put both front and rear light cables into the Shimano cable connector. It's very easy, the connector is just a plastic part that goes over the wires, so you can fit two wires then.
I don't think the "T" model is a good choice, though. There is no reason to ride around in daylight wasting valuable watts by having the lights on. The best models in my opinion are the ones that lack daytime running lights and only have an automatic light sensor to turn on both front and rear lights. The additional cost of the sensor isn't large. Another solution could be to have an ON/OFF switch in the front light that also controls the rear light. The sensor models usually have an ON/SENSOR/OFF switch allowing you to force the light on or off, or to use the sensor.