In many domains, models are created based on predefined metamodels which abstract the structure of the domain in question. However, there are specific domains, like Enterprise Architecture (EA) projects, where a metamodel cannot be defined in advance to the creation of the model. Unfortunately, in this situation using standard frameworks, like EMF, generates some inconveniences in the construction of the model and the metamodel because these frameworks do not support the manipulation of metamodels at runtime. In this paper, we propose a strategy to co-create metamodels and models in an incremental and simultaneous way. This proposal is supported by a dynamic approach that separates the linguistic and the ontological conformity concerns of metamodeling. This strategy has been implemented in a graphical editor called GraCoT, which also provides interactive assistance to guide the users during the co-creation process.