Q: Question about dual graphics card in macbook pro 15'

Hello,

Recently I have been looking to buy a macbook pro and I am someone who tends to play games quite a bit. I would love to buy an iMac, since that would be much better, but sadly I tend to travel a lot between Europe and Australia to visit friends, and do not always have space for an iMac. Mac mini would be a good choice but sadly has been disappointing for gaming recently and mac pro is out of my budget for now.

So I have been considering the latest macbook pro 15' with dual graphic cards and my question is - how exactly does it use both of these? The Geforce 750 is the most powerful of the two, but overall not a very high-end graphics card. It would work fairly well for me but considering the price I am not sure it is worth it. (I might not be able to run games well in 2 years or so and if I buy a high-end Apple computer I expect it to last for quite some time.) Do the dual graphics card actually help when it comes to perform one task with heavy graphics usage? Or can it only use one graphics card at the same time for one task?

Thanks in advance