Thermal conductivity through concrete is quite high, about 50 times that of air. Fourier's Law is used to calculate conductive heat transfer, which depends upon thermal conductivity of the material, the temperature gradient, and the surface area. So, if the temperature gradient between the two slabs is high, and there is a substantial area of contact, yes, heat will move readily from the heated slab into the unheated slab.
That said, if the unheated slab is in a heated room, the temperature gradient may well be small. In addition, if you provide a thermal break - even a narrow strip of closed cell foam serving as an expansion joint - then area equivalent for heat transfer is diminished and heat transfer from one slab to another becomes nearly inconsequential.
