I develop a theoretical model to investigate the effect of simultaneous regulation with a leverage ratio and a risk-weighted ratio on banks‘ risk taking and banking market structure. I extend a portfolio choice model by adding heterogeneity in productivity among banks. Regulators face a trade-off between the efficient allocation of resources and financial stability. In an oligopolistic market, risk-weighted requirements incentivise banks with high productivity to lend to low-risk firms. When a leverage ratio is introduced, these banks lose market shares to less productive competitors and react with risk-shifting into high-risk loans. While average productivity in the low-risk market falls, market shares in the high-risk market are dispersed across new entrants with high as well as low productivity.