  We show that the ability to make direct measurements of momentum, in addition
to the usual direct measurements of position, allows a simple configuration of
two identical mechanical oscillators to be used for broadband back-action-free
force metrology. This would eliminate the need for an optical reference
oscillator in the scheme of Tsang and Caves [Phys. Rev. Lett. 105, 123601
(2010)], along with its associated disadvantages. We also show that if one is
restricted to position measurements alone then two copies of the same
two-oscillator configuration can be used for narrow-band back-action-free force
metrology.
