If it has two inlets and two outlets then it is designed to use two pumps on two loops sharing a reservoir. If it has two out and two in but the pump can be directly attached to reservoir it is fine to use just one pump and it will be enough.
If you attach a pump in one loop after the reservoir, then the loop with no pump in it will have next to no flow, since there is no head/force to drive water through the other loop.**
Loops in series share radiators, so components which output the most heat (the ones you need to cool the most) will get a share of the cooling power proportional to their heat output. Because you are 'sharing' the coolant there wont be a big temp difference between the loops but a lack of flow in the loop with no pump attached will reduce the efficiency of the radiator and the block on that part of the loop.
Using the pump reservoir combo above will deliver temperatures much closer to a loop in series than a proper parallel loop but because one side of the loop will have different restrictions and therefore different flow to the other side of the loop, you may find that as you add components one radiator gets a bit warmer than the other for the same reason i mentioned above** where i described reservoirs without direct pump attachments combo'd with just a single pump, though the effect will be far less noticeable since the pump flow is distributed to both loops evenly rather than on just one loop (which is what your opening post sounded like)
*I didn't see the reservoir you were using before as the link wasn't working for me, but some google foo has got me a picture. Should be fine to use but to be honest, the set up your describing is practically a serial loop since the water will be shared and flow will be sufficient enough to keep water temps even.
