Once again, no, you can't. It would require, and I'm guessing but 99% sure, that you would need a better power supply for power, a logic board that was designed for dual 1.8 G5's, and the actual 1.8 G5 itself.
Why you would need a new logic board is that each logic board is designed for the processors that are going to be used with it. So the 1.8 has a logic board that has features/designs that are just for that certain type of processor. A dual 2.5 G5 has a logic board designed just for it.
The reason why there are those many fans is just a heat issue. Also, Apple has those cases pre-built to make them uniform so that each one is interchangeable with each processor class. This doesn't mean that a single 1.8 G5 can just be upgraded to a dual 1.8 G5. It just won't work without a different logic board, power supply, etc.
In my opinion, which means little really, it isn't worth the $$$ to try and do this. Especially now that a low end Mac Pro, or even a refurbished one, is $2499 or less. To do what you're suggesting would probably be around $1000-1800 and if you add just a little more you get a newer machine with Intel processors and a much better machine, IMHO.
*shrugs* Don't know how you feel but do what you want. I just hope it doesn't mess up your machine in the process. I liked those 1.8 G5's!
~Crawn