1. A computer-readable recording medium that stores therein a simulation program for controlling a computer which when executed on the computer performs a process for executing a plurality of core models in parallel, comprising: synchronizing an instruction being executed by a first core model and an instruction being executed by a second core model; causing the first core model to determine whether an access to the second core model is possible, and to access the second core model when it is determined that the access is possible; and causing the second core model to suspend the instruction being executed by the second core model, to execute an instruction from the first core model, and to resume execution of suspended instruction after execution of the instruction from the first core model is completed.