[rocketmanx]

I think the tank idea will work fine actually. Any "fighting" would be be one directional in that it may be fighting more "head" essentialy with the resulting pressure coming out of the tank at a sum of both but slightly less due to any extra resistnce from both pumps pressurizing the tank at the same time.
In series or parallel pumps "fight" each other and cause stress on both sides of the impeller. That's what will cause undue wear on the impellers.
I'm actually most curious about the benefits of 2 pumps really. Unless you're using weak pumps to begin with, holding what Cathar has recomended as pump overkill before to be true, then the advantage of 2 pumps would only be beneficial to those with weaker pumps. In which case 2 pumps could add up to the cost of a much better pump pretty easily. Plus you have the heat issue which is already been discussed and would only serve to be doubled by adding a second pump. Seems that may offset any benefit from flow rate increase.
I think redundancy would be the biggest attribute to running 2 pumps. In that case a series condition would be less benficial due to the fact that if one pump fails then the reamiining pump would be stuck driving the impeller of the dead pump. Parallel mode would seem better for redundancy sicne if one pump fails then it's essentially left with running the loop itself with no real ill effects from the dead pump.
For the cost I haven't quite understood why anyone would consider two pumps a benefit. Unless that person happens to have 2 pumps laying arond that are the same and of lower "power".
I'm curisous since I'm not sure I may not be missing something with the 2 pumps. Hopefully someone will answer that question.