I understand that you can't conduct a Twin Locator approach if one of the NDB's is u/s.
The design criteria for the minima on the approach are predicated on the additional tracking accuracy provided by the pair of NDB's used in conjunction with each other. The 1st NDB allows the a/c to be positioned more accurately for the commencement of final.
Of course there may be an alternative single NDB approach published using one of the locators, but that depends on the what the relevent authority decides. All other factors being equal, a single NDB approach will have a higher minima due to the lack of positioning accuracy when commencing final.
Rather a while since I studied this so any errors are purely my own!