Current through ambipolar carbon nanotube field-effect transistors (CNTFETs) can be controlled by two independent gates, enabling highly expressive XOR-based logic circuits. To promote efficient circuit design, it is important to develop an easy-to-use SPICE-compatible model for these dualgate ambipolar CNTFETs. This paper therefore introduces a closed-form model that matches the experimentally demonstrated behavior. This model is then applied for the first simulation of cascaded dual-gate CNTFET logic circuits that exploit ambipolarity for compact logic.