  The most massive galaxies in the present day universe are the giant
ellipticals found in the centers of rich clusters. These have old, coeval
stellar populations, suggesting they formed at high redshift, and are expected
to host supermassive black holes (SMBHs). The recent detection of several
high-redshift radio galaxies (HzRGs) at submm wavelengths confirms that indeed
some massive galaxies may have formed the bulk of their stellar populations in
spectacular dust-enshrouded starbursts at high-z. In this letter we compare
sensitive Chandra X-ray images - which identify actively-fueled SMBHs - and
submm observations - capable of detecting obscured activity in luminous
galaxies at high redshift - of the environments of three HzRGs. These
observations exhibit overdensities of X-ray sources in all 3 fields and a close
correspondence between the Chandra and SCUBA populations. This suggests that
both substantial star formation and nuclear activity may be occuring in these
regions. We identify possible pairs of Chandra sources with each of 2 SCUBA
sources, suggesting that their ultraluminous activity may be triggered by the
interaction of two massive galaxies, each of which hosts an accreting SMBH. The
presence of two SMBHs in the resulting remanent is predicted to produce a
flattened stellar core in the galaxy, a morphological signature frequently seen
in luminous cluster ellipticals. Hence the confirmation of pairs of Chandra
sources within individual, luminous SCUBA galaxies would provide additional
evidence that these galaxies at z~2-4 are the progenitors of the giant
elliptical galaxies found in clusters at the present-day.

Astrophysics