[lex.phases] p5

For a sequence of two or more adjacent string-literal tokens, a common encoding-prefix is determined as specified in [lex.string]. Each such string-literal token is then considered to have that common encoding-prefix.

[lex.phases] p5 only mentions two adjacent string-literal tokens, it didn't say either of them has an encoding-prefix. Hence, we should specify the complete situation in [lex.string] p7. The current [lex.string] p7 only explicitly specifies two cases

The common encoding-prefix for a sequence of adjacent string-literals is determined pairwise as follows: If two string-literals have the same encoding-prefix, the common encoding-prefix is that encoding-prefix. If one string-literal has no encoding-prefix, the common encoding-prefix is that of the other string-literal. Any other combinations are ill-formed.

It lacks the case

If two string-literals both have no encoding-prefix, they have no common encoding-prefix.