BikeCAD version 11 includes the long awaited tandem option. To start a tandem from scratch, click on the tandem icon 
to launch the tandem dialog box. Clicking the Add button will instantly create an additional seat tube, top tube, boom tube, and assortment of components. Notice that the seat tube length, seat angle, tube diameters and component selections are copied from the original single bike. It is therefore worthwhile spending some time establishing things like tube diameters and seatpost size on the single bike first so that you don't need to change them in both the front and rear after the tandem has been created.
Of course once you do have a tandem established, you can proceed to adjust dimensions to suit the unique requirements of the front and rear riders. Dimensions affecting the front end continue to be controlled as they always have. Saddle in the saddle dialog box. Handlebars in the handlebar dialog box etc. Controls affecting the rear end have been consolidated in the tandem dialog box. There are separate tabs for primary dimensions, tube specs, saddle, seatpost, cranks, pedals, stem, handlebars, seatpost clamp, rider and couplers.
Rear saddle height can be changed in the Saddle tab as well as in the Primary dimensions tab. Handlebars can be shifted up or down using the STMY input field. The length of the rear end is adjusted by changing the boom tube length with the BTX input field. In most cases, BTY will be set to zero. This will allow for the same bottom bracket height on all bottom brackets. If however, you choose to enter a non-zero value to allow for different bottom bracket heights, be aware that the height of the rear bottom bracket is determined by the settings in the Primary dimensions dialog box. All bottom brackets in front of the rear one are located relative to the bottom bracket immediately behind.
The display of components and other elements is still controlled using the display dialog box. Turning off the display of handlebars turns off the display of all handlebars. Turning on the display of the rider turns on the display of all riders. Meanwhile, each component within the tandem section has an eyeball icon associated with it. Turning the eyeball icon off here will only turn off the display of that particular component, not all components of that type.
Dimensions relating to all these new tandem elements can be displayed on the screen using the check boxes organized within the Tandem tab of the Dimensions dialog box.
There are no limits to the number of tandem sections that can be added. Clicking the Add button again will always add one more tandem section to the back of the bike. Each tandem section is controlled within its own numbered tab within the Tandem dialog box. Only the rear most tandem section will have a Delete button associated with it. Clicking the Delete button will remove the rear most tandem section along with all the unique dimensions assigned to it.
If you would like to use a continuous length of tubing for your top tube. You can click the Straighten top tube button in the top right corner of the Tandem dialog box. Clicking this button will pin the current position of the front and rear end of the top tube and will automatically adjust the seat tube height for all seat tubes in between so that the top tube will be continuous and straight all the way from front to rear. Clicking this button does not lock the top tube in this state. As you can see in this three person tandem, by shortening the rear seat tube, the front two top tubes remain straight, but the rear top tube is now no longer in line.
In many cases, you'll want to add additional tubes, to your tandem designs. This can be done using the Plus icon 
in the top tool bar. For more on adding additional tubes to tandems or single bikes see the video tutorial on adding extra tubes in BikeCAD.