We construct a theoretical model for a thermotropic ferronematic (magnetic liquid crystal colloid) in an asymmetric twisted nematic cell, subject to in-plane magnetic fields. At low colloidal concentrations, the nematic director (n) over cap is unchanged by strong fields, the magnetic director (m) over cap principally responds to its coupling to (n) over cap, and the suspension density peaks sharply where (m) over cap and H are co-aligned. At higher concentrations feedback between (m) over cap and (n) over cap aligns (n) over cap over a significant part of the cell, and the suspension divides into a region where (n) over cap is parallel to H, and a region where it is not. We also make calculations of the optical properties of this cell.