Oh, if you are talking about Kalka Shatabdi using HOG loco and 2 power cars, then the probable reasons are these :
1. Rake integrity - the one problem with rakes powered by a HOG loco or EOG cars is that rake integrity has to be maintained and swapping caoches in case of failures will be difficult because there the 3 phase High voltage cables that have to be connected between the coaches. So, usually a rake once decided will probably not be changed.
2. Loco availability - This is specific to...
more... the HOG loco + EOG cars problem you stated. First thing is that we don't have dedicated power cars in IR yet ( 1 power car will be needed for HOG trains). So as of now, EOG cars are being used for the distribution of power from the loco ( the diesel generator won't be running). Now in case a HOG loco is not available for running the train and it has to be run with non-HOG loco, then both the EOG cars will be required again. So to avoid changing the rake daily, they are probably just using the same rake all the time with 2 EOGs.
Backup is only one of the reasons for using 2 EOGs. The other reason is that while a single EOG car is capable of handling a 18 coach rake, peak efficiency is usually attained at 12-15 coaches. That's why 2 EOGs are used so that based on the load requirements only one can power the rake or both can share the load if load increases ( A/C loads are affected by external weather conditions too). Once HOG becomes mainstream, this can be abandoned. As of now, full freedom is possible with the rakes only for Self-generating coaches.