What makes a good CB duo?

I am playing a CM with Roma and am fed up with Juan and Mexes. In the coming offseason I intend on buying two new CB and was wondering if you guys had some suggestions as to what makes a good pair. Maybe there isn't anything but you never know unless you ask.|||I like having a taller CB with good heading as LCB and a faster one as RCB. One of my favorite pairings is Pique and Lovren. They can switch out and they both bag a lot of goals especially from set pieces. You either need a really tall or really fast CB. That's my opinion anyway.|||Thanks, I am looking at Douglas from Twente. So he will take care of the height.|||Thanks, I am looking at Douglas from Twente. So he will take care of the height. Any suggestions on good but cheaper fast CBs?|||Cheaper? I guess you can check any of the premier weaker premier league's defensive lines. There's also a couple of CDM's that double as a CB so you can also check those. One of those is Toulalan of Lyon. He's a great tackler but I'm not sure about his speed. But you'll have to do some searching. Maybe in the search try and find defenders with the speedster trait?|||I guess cheap isn't the right word. Mainly just guys outside of the top tier crazy expensive CBs.|||Yea, a decent CB will cost you at least 20m unless you want to grow your own.|||I like using Mexes and Astori.|||I use Hangeland (6 ft 5) and Metzelder (6 ft 4) for Schalke 04, no one can beat them in the air and I score almost every corner|||i also like a strong defender (or stocky as their body type) and a tall, fast one at right back. I use Gabbidon and Baird (ok not that tall) and there good together but their overall's are only in the 70's and their both like 35!|||I have Pique and David Luiz at my Barcelona one. But if you go to Bremen, Naldo and Mertesacker are like 6 ft 6 each :)
There's a few really fast CB's in the lower leagues, one even has 93 sprint speed, not sure about his taclking though|||If u have the money Tiago Silva & Chiellini|||I find that that a tall and fast pair is good as well. Next season I am thinking of buying a fast CB to match the 6ft 4 Kroldrup :)|||Manoel is a great CB, strong, fast good headers and cost around 5-6 milion euro|||on my ultimate team it is david luiz---carvahlo they do well together