I was looking into making a water cooling loop for my computer and I watched a LinusTechTips video about it. He used 1 rad for his CPU and 1 for sli graphics (590's). But I have a smaller case and don't have space for two rads and seeing as I'm not using a sli dual gpu cards I was wondering whether I would be ok with one dual rad?
The amount of radiators doesn't matter; you could use 10x 80mm radiators for a triple SLI or I could use 4x 230mm radiators. What is important is radiator space; if you use tons of tiny rads or one super massive rad, it is all the same.
More over, you have to account for radiator thickness, fin density, how many passes they are (double, single, etc.), and what fans you are using. For a single GPU and CPU, 240mm, 280mm, or 360mm of total rad space would be fine.
The things you need to consider when selecting the right amount of radiator space is first:
Will it fit in the case.
What are your ambient temperatures and how much does this vary from your case temperature.
What configuration are you going to put your fans in. (push/ pull, push or pull and exhaust or intake)
Fans can also make a difference in terms of how the radiator performs. I suggest purchasing fans that are actually suited for water cooling and for use on radiators then ones that are specifically designed for airflow.
I hope this helps to answer your questions and that I didn't actually add to your confusion.