Why'd you have to complicate things? :P
At Barts there are two SHO tiers, one lot has both degrees, one lot has just one (either). There is one guy from each tier on at the same time. The double degree guys will usually be career MaxFax, while the one degree guys are more likely to be passing through.
Usually the locums will have worked in the dept in the past (as SHOs) and the bosses know what they can and can't handle. They will not get on proper SpR rotations but can do ad hoc SpR shifts if the bosses agree (more money, better experience). I have a medic mate who does exactly that.
Dunno about it changing, official rules may well be... but since it is such a competitive speciality you'd struggle to get on a rotation with less than your peers (which will be a BDS+MBBS in most cases).