[spaghetti]

No I don't think you should mount two bands: more power reduce accuracy and the gun is harder to swing.
So my word is no gospel, but if you ask me, for a 75 euro this is a correct set up with a good power and punch:
band: one circular band, 20 millimeters thick, 42 centimeters long
shaft: single flopper tahitian, 115 centimeters long, 6,5 millimeters thick.
---
For the power thing, note that the average 75 euroguns are sold with 6 or 6,3millimeters shaft and 16 or 18 millimeters band. So if you decide to follow my humble suggestions and mount 20mm bands and 6,5mm shaft your gun will have more "punch" than the average for this category of gun.