Well here goes... Aluminium frame router design

[Boyan Silyavski]

I would use one belt only, as its easier to align properly. Make sure the idler pulleys not to bend. I would encase them so every steel pin is supported at both sides. Makes one of the idles a tensioner. Find the Gates belts site and there were calculations how to tension the belt using a guitar tuner if you dont have proper device to measure the tension.
Mistakes here will affect the performance, especially the precision. Belt must be tensioned properly, not by feel.