This paper proposes a frequency-constrained dispatching method for integrated electricity-heat microgrids (IEHMs). It employs synergic primary frequency regulation (PFR) resources to arrest the frequency drops. First, the frequency security problem in IEHMs is described. Combined heat and power (CHP) unit and battery storage coordinate to PFR provision. The frequency nadir in the dynamic frequency evolution is modeled as second-order cone constraints. The total PFR provision is restricted considering quasi-steady-state frequency. Second, a frequency-constrained dispatching model for IEHMs is presented based on the frequency security constraints. PFR reserve capacities of CHP unit and battery storage are determined to secure frequency security and reduce the PFR reserve costs. Wind power uncertainty is described by a distributionally robust chance constraint. Finally, the effectiveness of the proposed method on the frequency security and economy is validated in case studies. The impact of heat load on PFR in IEHMs is also revealed by numerical tests.