Is it possible for Windows Workflow wca.exe tool to include more than one assembly?

Question

I would like to create an interface that references my business objects for a Windows Workflow, but when I use the wca.exe tool to create the Invokes and Sinks files, I receive an error stating that the assembly for the business object can not be found.

Here is my a snapshot of my code to further explain:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Workflow.Activities;
using BusObj = REIMSWorkflowBusObj;

namespace REIMSWorkflowBL.WorkflowEventService
{
    [ExternalDataExchange]
    public interface IWorkflowEventService
    {
        void UserTask(Guid workflowInstanceGuid, 
                            int workflowInstanceId,
                            BusObj.WorkflowTask workflowTask);
        event EventHandler<WITYApprovalArgs> Response;
    }
}

The error message is:

Processing interface 'REIMSWorkflowBL.WorkflowEventService.IWorkflowEventService' Generating InvokeMethod for 'UserTask' ERROR: Exiting because of exception: System.IO.FileNotFoundException: Could not load file or assembly 'REIMSWorkflowBusObj, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. The system cannot find the file specified.

I have tried copying the assembly to the directory where the interface source code resides, and get same results. Any thoughts?