Component1:

<!--Component1-->
<aura:component implements="force:appHostable,flexipage:availableForAllPageTypes" access="global" >
    <div>
        Component 1   
    </div>     
    <lightning:button variant="brand" label="Navigate to Component 2" onclick="{!c.navigate}" /> 
</aura:component>

Component1 Controller:

({
    navigate : function(component, event, helper) {
        var navigateEvent = $A.get("e.force:navigateToComponent");
        navigateEvent.setParams({
            componentDef: "c:Component2"
            //You can pass attribute value from Component1 to Component2
            //componentAttributes :{ }
        });
        navigateEvent.fire();
    }
})

Component2:

<!--Component2-->
<aura:component implements="force:appHostable,flexipage:availableForAllPageTypes" access="global" >
    <div>   
        Component 2
    </div>    
    <lightning:button variant="brand" label="Navigate to Component 1" onclick="{!c.navigate}" />
</aura:component>

Component2 Controller:

({
    navigate : function(component, event, helper) {
        var navigateEvent = $A.get("e.force:navigateToComponent");
        navigateEvent.setParams({
            componentDef: "c:Component1"
            //You can pass attribute value from Component2 to Component1
            //componentAttributes :{ }
        });
        navigateEvent.fire();
    }
})