In your case, you have a single book authored by two people with the same surname but different initials.
Your in-text citation would look like this:
As P. Ehrlich and Ehrlich underlined, . . . (2015)
And your reference list entry would look like this:
Ehrlich, P., & Ehrlich, A. . . .
The explanation for this follows.
The Publication Manual of the American Psychological Association (6th ed.), 6.14, says this about reference lists:
If a reference list includes publications by two or more primary authors with the same surname, include the first author's initials in all text citations, even if the year of the publication differs. Initials help the reader to avoid confusion within the text and to locate the entry in the list of references.
Light, I. (2006). Deflecting immigration: Networks, Markets, and regulation in Los Angeles. New York, NY: Russel Sage Foundation
Light, M. A., & Light I. H. (2008). The geographic expansion of Mexican immigration in the United States and its implications for local law enforcement. Law Enforcement Executive Forum Journal, 8, 73—82.
Among studies, we review M. A. Light and Light (2008) and I. Light (2006).
Note that the in-text citation for the coauthored book uses the first author's initials and surname, but only the second author's surname. This matches your own example.
Here is some additional information.
Arrange works by different authors with the same surname alphabetically by first initial:
Mathur, A. L., & Wallston, J. (1999).
Mathur, S. E., & Ahlers, R. J. (1998).
If the reference list includes different authors with the same surname and the first initial, the authors' full first names may be given in brackets:
Janet, P. [Paul]. (1876). La notion de la personnalite [The notion of personality]. Revue Scientifique, 10, 574–575.
Janet, P. [Pierre]. (1906). The pathogenesis of some impulsions. Journal of Abnormal Psychology, 1, 1–17.
(Paul Janet, 1876)
(Pierre Janet, 1906)