"Whom" or "who" where the referent is both subject and object?
Note how the original is constructed:
Neither of these objections applies to the version of contractualism that I am defending. The general specification of the scope of morality which it implies seems to me to be this: morality applies to a being if the notion of justification to a being of that kind makes sense.
The author is dealing with some pretty complicated thoughts here; but he has managed this fairly gracefully† by breaking his argument into easily processed chunks.
First, he divides it into two stages: a. “What I'm talking about: I'm going to specify the scope of morality [with some necessary hedges]”, and b. “What I have to say about it: this is the scope”.
In stage b. he redefines scope as an active verb, applies, and cunningly employs a conditional construction to distinguish b1. the act of applying morality from b2. the entities to which morality may be applied—taking care, however, to knit the two together by repetition of the term being. This is critical: because that separation allows him to use the preposition to for the complement of two different lexemes, applies and justification, without confusion.
Now you should be able to see where your attempt at a paraphrase has tripped you up.
In your subject you have tried to embrace both scope of morality and morality applies to—which are the same thing: the scope does not apply to beings: the scope is the beings. Consequently you get your prepositions (of, to) and their objects tangled up.
In the object of restricted to you have recast the first being as a plural those, and defined that with a relative who/whom which has no distinct syntactic role to play in what follows. You are trying to relativize an entity which plays two different roles. Moreover, your prepositions are all cattywhompus because that entity stands as the object of two different tos (in addition to the to of your restricted to!), and you have lost the conditional construction which distinguishes them. (Even if you had kept the conditional construction you would still need another pronoun; see the comments to this question by our syntax guru John Lawler.)
It would be much simpler to let Scanlon speak for himself:
Scanlon restricts his account of morality: “morality applies to a being if the notion of justification to a being of that kind makes sense”.
†I have some very minor cavils. I'd handle the first clause with fewer nouns, thus: "It seems to me that the scope of morality which it implies may be generally specified thus:...". And justification to is tricky; it seems at first glance to be an error because it is immediately after this point that Scanlon explains what it means. I would bracket it somehow: justification-to or 'justification to'.