In addition to single inlet ports of the types described, dual ports of various designs are also being used. One arrangement is the combination of a peripheral port and side port (combi-port system); another is the use of dual side ports, one in each end cover. Both ports may be served by the same manifold, or separate manifolds may be used. In the latter case, one preheated (by exhaust gasses) manifold, which is unobstructed at all times, leads to a side port and ensures good starting and low-speed performance. The second, a peripheral-port manifold, is closed or opened by a suitably controlled throttle valve and serves only to furnish the additional amount of mixture needed under higher-speed / greater load operating conditions.

When using a peripheral exhaust port and a single side port, overlap may be approximately 45% of 270 degrees of mainshaft rotation, or with a single peripheral-inlet port, it may be approximately 74%. Therefore, use of a constant-open side port together with a throttle-controlled peripheral inlet port provides a desirable low-speed overlap down to 45%, which is increased to about 74% when both ports are in use for high-speed operations.