I don't understand the low dielectric constant point. If you join PCBs, the dielectric would be formed by the PCB substrates, a thin adhesive layer won't change it. The adhesive should be chosen according to the substrate material, e.g. epoxy for FR4, microwave substrate may need different adhesive for sufficient bonding.
Alternatively, if you want a defined dielectric layer between the substrates, you probably use a rigid substrate instead of a thick adhesive layer. Or may be foam in some cases. The layer thickness should be well defined, though.
A drawing might explain the problem better.