  We use quantum Monte Carlo (stochastic series expansion) and finite-size
scaling to study the quantum critical points of two S=1/2 Heisenberg
antiferromagnets in two dimensions: a bilayer and a Kondo-lattice-like system
(incomplete bilayer), each with intra- and inter-plane couplings J and J_perp.
We discuss the ground-state finite-size scaling properties of three different
quantities--the Binder moment ratio, the spin stiffness, and the
long-wavelength magnetic susceptibility--which we use to extract the critical
value of the coupling ratio g=J_perp/J. The individual estimates of g_c are
consistent provided that subleading finite-size corrections are properly taken
into account. In the case of the complete bilayer, the Binder ratio leads to
the most precise estimate of the critical coupling, although the subleading
finite-size corrections to the stiffness are considerably smaller. For the
incomplete bilayer, the subleading corrections to the stiffness are extremely
small, and this quantity then gives the best estimate of the critical point.
Contrary to predictions, we do not find a universal prefactor of the 1/L spin
stiffness scaling at the critical point, whereas the Binder ratio is consistent
with a universal value. Our results for the critical coupling ratios are
g_c=2.52181(3) (full bilayer) and g_c=1.38882(2) (incomplete bilayer), which
represent improvements of two orders of magnitude relative to the previous best
estimates. For the correlation length exponent we obtain nu = 0.7106(9),
consistent with the expected 3D Heisenberg universality class.
