  We identify SDSS J130033.48+590407.0 as a common proper motion companion to
the well-studied DA white dwarf PG 1258+593 (GD322). The system lies at a
distance of 68 +/- 3 pc, where the angular separation of 16.1 +/- 0.1 arcsec
corresponds to a minimum binary separation of 1091 +/- 7 AU. SDSS J1300+5904 is
a cool (Teff = 6300 +/- 300K) magnetic white dwarf (B ~ 6 MG). PG 1258+593 is a
hydrogen-rich (DA) white dwarf with Teff=14790 +/- 77 K and log(g) = 7.87 +/-
0.02. Using the white dwarf mass-radius relation implies the masses of SDSS
J1300+5904 and PG 1258+593 are 0.54 +/- 0.06 Msun and 0.54 +/- 0.01 Msun,
respectively, and therefore a cooling age difference of 1.67 +/- 0.05 Gyr.
Adopting main-sequence life times from stellar models, we derive an upper limit
of 2.2 Msun for the mass of the progenitor of PG 1258+593. A plausible range of
initial masses is 1.4 - 1.8 Msun for PG 1258+593 and 2 - 3 Msun for SDSS
J1300+5904. Our analysis shows that white dwarf common proper motion binaries
can potentially constrain the white dwarf initial-final mass relation and the
formation mechanism for magnetic white dwarfs. The magnetic field of SDSS
J1300+5904 is consistent with an Ap progenitor star. A common envelope origin
of the system cannot be excluded, but requires a triple system as progenitor.
