An electronic device may include a millimeter wave transceiver, a first antenna having a first resonating element at a first side of a substrate, and a second antenna having a second resonating element at a second side of the substrate. A first coplanar waveguide may convey millimeter wave signals between the transceiver and the first resonating element and a second coplanar waveguide may convey millimeter wave signals between the transceiver and the second resonating element. The first coplanar waveguide may be coupled to the first resonating element through the second coplanar waveguide. The second coplanar waveguide may be coupled to the second resonating element through the first coplanar waveguide. Ground conductors in the coplanar waveguides may form antenna ground planes for the first and second antennas while serving to maximize electromagnetic decoupling between the coplanar waveguides and thus isolation between the ports of the transceiver.