Pentax SFX flash query

Now I know this is a bit ancient but bear with me - I've just loaded a roll of slide film into this '80s monster and had a thought. The flash hotshoe on the SF series is off to the right hand side above the grip, meaning that you can deploy the onboard flash at the same time as having a detachable one fitted.

If you use both (with the additional flash in TTL mode) what happens?

You can use them together in TTL Auto flash mode.

Page 47 of the manual says so.

Manual password is "Pentax" without quotes and case sensitive.