peace Marie,
I'm not entirely sure what you mean, but I will try to clarify...
In 11:114, 'zulafun' is plural (i.e. three or more near/close/adjacent parts of the night). Now, lets use the minimum criteria for plural usage (i.e. 3), this 3 adjacent parts are to be used for the 2 salat (i.e. fajr & isha).
One example...
The time-range of fajr-salat = one adjacent part of night (twilight) until full sunrise (i.e. sun is fully above horizon)
The number of near/close/adjacent parts of the night used for fajr-salat = one.
The time-range of isha-salat = from rubbing of the sun (i.e. part going below the horizon), including twilight (one adjacent part) AND early night (another adjacent part).
You see, whatever salat is, one may be so focused that one doesn't realise the time and you enter the early part of the night (this is after twilight) THUS the plural criteria (i.e. 3 near/close/adjacent parts have been used altogether for the 2 salat). One adjacent part for fajr and two for isha, giving three.
Correct me if I'm wrong, tarafay alnnahar means the sunrise (the begining) and the sunset (the end).
As far as I know, taraf = extremity, edge, lateral/adjacent/outward part, side, border ,end, newly acquired. Used in verse 11:114, it does not specifically mean sunrise and sunset. It just means the edges (two) of daytime/daylight. The 'edges' need clarified, hence 17:78.
So, as you can see, 17:78 AND 11:114 are needed together for clarification and to confirm a time-range.
I hope thats helped...
Wakas