Simple Past Tense for duration of time

The simple past implies the action took place in the past and is now finished.

If the action (conversation) is still in progress, then I think "Sarah and David had been talking for two hours" would be better.

The simple past tense is used to express a completed action in the past usually with a reference to a definite time.

Sarah and David talked for two hours yesterday after school.

Where as the present perfect indicates basically an action that has just been finished or is likely to continue even after the time of speaking.

Sarah and David have talked for two hours, now.

The past continuous however, is used to say that one action in the past was in progress when another action took place.

When I entered the class Sarah and David were talking.

We use for + duration in different tenses :

• Sarah and David talk for two hours daily. (Regular habit).

• Sarah and David have talked for two hours. (They have just completed their talking.)

• Sarah and David talked for two hours. (Their talking ended in the past.)

• Sarah and David were talking for two hours. (They were talking for two hours and it ended in the past.)

• Sarah and David had been talking for two hours before I came (or, when I came). (to say how long they had been talking up to a past moment.)

• Sarah and David will not talk for the next ten days. (They won't talk for a period in future.)

*** But to measure "DURATION UP TO THE PRESENT", we use a present perfect tense :

• Sarah and David have been talking for two hours. (It means their talking started in the past and is still going on.)