Can A Billon Antoninianus Have The Same Ric #as A Bronze One

As far as I know they are one in the same. The coins are not billon but are bronze or a base metal with a silver wash.

Short answer: if you could find an issue in both billon and bronze they would have the same RIC number.

The problem you are raising has to do with the so-called "fabric" of the coins. The alloy from which they have been made. The antoniniani of Gallienus, like their predecessors and successors, are all regarded as silver coins regardless of the changes in fabric due to debasement. RIC V only lists by obverse, reverse, and date, providing citations to a published authority for any special features (including the date). Typically even the most debased coins were issued with a silver wash to give the appearance, no matter how fleeting, of being a silver coin. But the fabric is useful in dating the coins, so a specialist might attempt to arrange the coins of Gallienus by their silver content as an indication of relative dating.

During the reign of Gallienus the antoninianus underwent an accelerated rate of debasement, and by degrees the amount of silver in the coin was reduced until the coins consisted of a bronze core with a light silver wash. Silver coins of the US were similarly debased in the decade of the 1960s, so that by the end of the decade a cupro-nickel "clad" coin had replaced what had been made of a silver alloy. But the new coins were tariffed at the same rate as before.