  We use neutron scattering to study the electron-doped superconducting
NaFe$_{0.985}$Co$_{0.015}$As ($T_c=14$ K), which has co-existing static
antiferromagnetic (AF) order ($T_N=31$ K) and exhibits two neutron spin
resonances ($E_{r1}\approx 3.5$ meV and $E_{r2}\approx 6$ meV) at the in-plane
AF ordering wave vector ${\bf Q}_{\rm AF}={\bf Q}_{1}=(1,0)$ in reciprocal
space. In the twinned state below the tetragonal-to-orthorhombic structural
transition $T_s$, both resonance modes appear at ${\bf Q}_{1}$ but cannot be
distinguished from ${\bf Q}_{2}=(0,1)$. By detwinning the single crystal with
uniaxial pressure along the orthorhombic $b$-axis, we find that both resonances
appear only at ${\bf Q}_{1}$ with vanishing intensity at ${\bf Q}_{2}$. Since
electronic bands of the orbital $d_{xz}$ and $d_{yz}$ characters split below
$T_s$ with the $d_{xz}$ band sinking $\sim10$ meV below the Fermi surface, our
results indicate that the neutron spin resonances in
NaFe$_{0.985}$Co$_{0.015}$As arise mostly from quasi-particle excitations
between the hole and electron Fermi surfaces with the $d_{yz}$ orbital
character.
