Comparing the mesomeric effects of amino and mercapto groups
For a mesomeric effect to occur, an orbital has to align with the &#x3c0; system it is supposed to resonate with. Since &#x3c0; systems have a defined $z$ axis — the orientation of the $\mathrm p_z$ orbitals that make up the &#x3c0; system — only orbitals that also feature such an alignment an interact. This limits any mesomeric interaction of any heteroatom with a &#x3c0; system to that heteroatom’s $\mathrm p_z$ orbital. Therefore, both nitrogen and sulphur can only have one orbital at their disposal for $+M$ effects.

The reason why the $+M$ effect of nitrogen is stronger despite its higher electronegativity is the period these elements are in. Nitrogen, being in the same period as carbon, has orbitals that fit together well size-wise with carbon orbitals. This is also reflected by the ease of forming $\ce{C=N}$ or $\ce{N=O}$ double bonds. Sulphur is one period down. Its orbitals are more diffuse than carbon’s or nitrogen’s; therefore, any &#x3c0; overlap is much weaker. This is also the reason why $\ce{S=O}$ double bonds are rare and $\ce{S=C}$ double bonds are typically more reactive than $\ce{O=C}$ double bonds.