Having thought it through a bit, Google helped and I found this article on the FamilySearch Wiki that is adapted from a print article by respected genealogist, Anthony Camp. Although it is about bonds on marriage licences and the original question is about probate bonds in particular, I see no reason why what applies to one bond type shouldn't apply to them all.
The relevant bits are
The bond, sworn "by two sufficient witnesses" ... pledged to forfeit a large sum of money (ranging from £40 to £200), if there was any consanguinity (a relationship within the prohibited degrees) between the parties or any pre-contract to another person. ... The second bondsman soon became a formality, any convenient person acting. Later the second bondsman was often completely fictitious, names like John Doe and Richard Row being used.
My emphasis in italics. Thus it suggests to me that there was indeed a requirement for two bondsmen but the procedures around the second bondsman became somewhat informal, reducing, at the extreme, to fictitious names.
Personal interpretation - the risk associated with a fictitious second bondsman is surely not material. The objective of the bond is to concentrate the mind of the bondsmen on ensuring due process is followed on the marriage or probate. By having only one real bondsman, the effect is that one guy is doubly incentivised. The only risk is if (a) probate or marriage goes wrong and (b) the first bondsman is dead or cannot otherwise pay. Presumably the Church authorities would not accept a bond signed by a pauper in the first place so would need to feel some sort of confidence that the bondsman could pay and that this was therefore a genuine deterrent.