We have investigated the magnetoresistance properties of mesoscopic wires fabricated from a modulation-doped GaAs/(AlGa)As double quantum well structure containing two coupled 2D electron gases (2DEGs). An in-plane magnetic field, B, “tunes” the tunnelling between the 2DEGs and, in a Hall bar device, leads to a resistance feature at B≈10T due to a van Hove singularity in the density of states. In mesoscopic wires this feature becomes a strong resistance peak. We attribute this enhancement to a size effect. In addition, we observe universal conductance fluctuations with the in-plane B. These disappear when B is strong enough to suppress the tunnelling between the wells.