A number of software that supports multiple interactions such as synergistic use of speech and gesture is increasing. With power and versatility of multimodal interfaces also comes and new issues like increased complexity of software design. These issues should be considered in early design and testing stages of software development process. In this work we are researching a complex use of users multiple interactions with software. We will represent a method for the specification of multimodal user interfaces offered by other authors. This method is based on CARE properties. The CARE properties provide a formal framework for reasoning design of multimodal systems. Multimodal systems are using fusion mechanisms to combine different modalities. Later in this work we will take a closer look at testing process of multimodal applications. A new approach for testing modalities fusion mechanism in multimodal applications is offered. This approach will be experimentally demonstrated, evaluated and compared to alternative method.