Zero-range distorted-wave Born approximation calculations are used to investigate the systematics of two-nucleon transfer reactions. The shapes of the calculated 40Ca(t,p)42Ca(gs) and 208Pb(p,t)206Pb(gs) cross sections are relatively insensitive to details of the core plus two-neutron form factor, if the form factor tails have similar shapes. Studies of heavy targets such as 206Pb suggest a surface reaction mechanism, but lighter targets (16O and 40Ca) indicate that spatial regions beyond the target surface provide the dominant contribution to the (p,t) cross section.