If both of the fathers are recognisably male due to having black backs then they must be adults (young birds will be pale grey-backed until the post juvenile moult in autumn).
Mixed pairings like this occur in several species (infamously - the dunnock) and such behaviour is probably overlooked. During research on farmland birds, I've seen at least three cases of female yellowhammers rearing broods with the help of more than one male: two nests with two males and one with three! I've watched a couple more nests where I could not prove this was happening, but where the rate of nest visits was way too high for just two parents.
In the case of yellowhammers, the females appear to recruit extra help by soliciting copulations from multiple males, even when she is not laying eggs. Presumably this tricks the extra males into thinking that they have a genetic investment in the broods. In contrast, cirl bunting females only copulated during egg laying and we never saw a single instance of polyandry.
