From tomestone and artifact evidence we're figuring the 2-belt style was popular from Augustus up maybe the Flavian period, notably with maille and scale-clad Romans.
From what we can tell from limited sculptural evidence, notably Domitius Ahenobarbus altar - that soldiers in the [Late] Republic were wearing only a single belt, with no apparent plates or decorations. It also "appears" (to me at least) the Republican belts were wider than Augustan 2-belts, being c. 2"+ wide, and seem to show (on Ahenobarbus) the belt with incised lines on the top and bottom edges. Whereas the Augustan belts appear to be c. 1" wide, with 1" X 2" belt plates, give or take on specific finds/designs. Then the "Imperial" belt, with c. 2" wide belt, c. 2" square plates shows up.
Also on the Aemilius Paulus monument, the soldier appears to also have this single, c. 2"+ wide belt, one can see the hole where a sword scabbard had been placed. It also appears there may be a little detail next to that hole that may point to where the buckle may have been, that, or it's just damage to the sculpture over time.
IMHO, the absence of belt frogs I think indicates it's possible that frogs were not used/seen until after Caesar ~ the sword (and dagger) may have been simply tied onto the belt from the suspension loops on the scabbards. Again on the Ahenobarbus altar, the soldier with his hand to his helmet, you can see his gladius on his belt, but I see no surviving detail pointing to any frogs, instead it looks like it's pressed right against the belt and to his side, which to me indicates it's strapped onto the belt somehow (like with a X with leather ties) Of course, that level of detail may not have been in sculpture in that period, or, any detail that was there has long since broken/fallen off.
You can see the Ahenobarbus and Paulus sculptures in Osprey's "Republican Roman Army - 200-104 BC" by Nick Secunda (Men At Arms 291)
also try here on Romanarmy.com, look under "Resources Gallery", and then "Sculpture"
http://www.romanarmy.com/cms/component/ ... temid,135/
http://www.romanarmy.com/cms/component/ ... temid,135/
Also, look on Matt Amt / Leg XX page for his idea on the belt, with a Glad. Hispaneis:
http://www.larp.com/legioxx/hispan1.jpg
(hope you don't mind me posting, Matt)