No. The closest you could get would be,
h = plot(largebinsmesh, guessCurve, 'k-', largebinsmesh, guessCurve,'w-.','linewidth',1.2);
set(h(1), 'linewidth', 3.0);
This presumes that guessCurve is a vector; if it is a 2D array then it would be h(1:size(largebinmesh,2)) that would have to be set()
plot() accepts multiple string specifiers for the plotspec but it does not accept multiple parameter value pairs for the same parameter name.