Thank you for all your inputs.
As I mentioned earlier, we have 3 Hrs movie and we have to insert the Ad just before particular scene OR after particular scene. So, if we use 10 secs segment, we can insert Ad at 10 secs, 20 secs, 30 secs, 40 secs. and so on (only at multiples of 10). There is no way to insert the Ad at 25th second. If we place the ad at 20th second or 30th second, it will be very early before the scene OR very late after the scene. That is why, we tried with 2 secs segments.
I am just waiting for Smooth Streaming support on ROKU... Hope it will support Composite Manifest feature.