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What makes mammals tend to evolve to have two testes?

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My guess would be symmetry and redundancy. :-) – MCM Sep 25 '12 at 4:15
I guess the same could be asked of ovaries – harpalss Sep 25 '12 at 8:14
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And, you could probably ask the same about: ears, eyes, nipples, etc. – Steve Lianoglou Sep 25 '12 at 17:03
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Two eyes and ears is to do with stereo receptivity, having two sensors of the same type in different places helps to gather much more information about distance and position (triangulation) of the source. One cool example of this is the hammerhead shark. It's believed that it has smell sensory organs in either end of its head. The further apart they are the better the sharks are at finding prey. It is also why predators have the eyes on the front of their head - so they work in tandem. news.bbc.co.uk/earth/hi/earth_news/newsid_8376000/8376740.stm – rg255 Nov 7 '12 at 13:48

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EvoDevo is not my field, but I will try to give you some pointers.

Mammals as vertebrates, start out as fertilized oocytes, transform into a rapidly dividing lump of cells with eventual polarity through numerous gradients of numerous chemical messengers, and form a neural tube. The forming body is ever more segmented through the use of gradients and thresholds (see french flag model) and (dorsoventral) axial symmetry is maintained throughout development, and as such two testes represent the default development.

Therefore, it may be more interesting to ask for singular organs which require more information, and thus maintenance throughout evolution of this information. But when considering the energy requirements, the need for synchronization, the need for a separate blood supply and thus the introduction of additional faults, it is easy to see why we only have one heart.

Wikipedia; author: Zephyris Wikipedia; author: Zephyris, title: "The first few weeks of embryogenesis in humans", DOR: 25/09/2012

See:

I hope this helps as a starting point.

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I might further speculate that the female reproductive system has strong axial symmetry and the testes are probably following the conservative development pattern of the female reproductive system. – shigeta Sep 25 '12 at 18:18

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