It really depends on how you use them and in what environments. This is a broad subject, but some overview set ups are below. I will be glad to go into specifics if needed.
Mixing TTL and manual flash is typically not recommended, as you will run the risk of over exposure in most cases. However, it can be done if done in certain circumstances (i.e. the two are lighting different areas of the image, with little to no overlap).
In this case, you can keep the speedlite in the hotshoe and plug the Einstein in via the sync cord into the PC port of the camera. If your camera does not have a PC port, you can use a pass through hotshoe adapter like this one:
http://www.flashzebra.com/products/0268/index.shtml . Also, you can use a remote system, like CyberSync, and plug the remote into the PC port, as described.
Alternatively, you can use the flash in manual mode, dialing in the appropriate power into the back, then letting the Einstein slave off the flash of the speedlite. You can also still do this with a remote system as well instead of relying on optical slaves.
You can also put the speedlite in A mode. This will provide a level of automation in the speedlite, yet still allow the Einstein to slave properly. Again, you can also use radio remotes.
The main thing is to not try to slave the Einstein when the Speedlite is in TTL mode. TTL will emit a preflash, which will cause the Einstein to trigger before the exposure, and likely not be recycled and ready to fire for the second flash during the actual exposure.