Hi,
I think its a very bad idea. First no upgrade will get you Dual G5 performance because the bus is slower.
Second if you ever have problems with your system you will always have to wonder if its the upgrade which is not unlikely.
Third the savings are not going to be that big. Refurbed Dual G5’s go for about $1,400. You can probably sell your system on Ebay for $200 to $500. How much will you save after the cost of the upgrade? Now figure in resale value. A Dual G5 is going to be worth quite a bit more in the future than a tricked out G4.
Small savings, risky compatibility and a slower system.
Best Wishes,
Mitch