Indian railways apply a restriction to the total weight of your luggage in kilograms. You can see this as per the luggage rule. For your class of travel, the limit is 50 Kilograms. Though there are dimension limits, I think being standard size your luggages should not exceed them:
Trunks, suitcase and boxes having outside measurement of 100 cms. x 60 cms x 25 cms (length x breadth x height) will be allowed to be carried in the passenger compartments as personal luggage
As far as you are within this limit you are allowed to carry it within the compartment.
There is also a maximum weight that you can take as "luggage" (which in Indian Railway technical terms is personal luggage, the other type of goods carried in a passenger carrying Train being named "parcels" which do not need an accompanying passenger and is not guaranteed to be transported by any given train/route) within the compartment per head which is 100 Kilograms per passenger in the class of travel you indicated.
Anything above free allowance has to be paid for. In case if you have not paid and you are found to be carrying extra luggage you will be penalised. Anything above the maximum permitted luggage has to be booked in brake van (a goods carrying coach attached to almost every express/passenger train in Indian Railways).
Coming to your specific case, to be perfectly safe you can "book" your luggage in a break-van which is guaranteed to be transported in the same train as yours, if you feel this would cause problems inside the coach. You will have to pay for it and pack it appropriately to ensure safe passage, however you are guaranteed of it causing no issues to your travel. If you want to be perfectly safe you could disembark quickly at your destination and rush to the luggage compartment to ensure that the package is not mishandled by the staff or porters. Moreover, doing so you would be ensuring that your luggage not over-carried.
Also note that in each AC 2-tier coupé (which is a sub division of the coach) there are only six passengers, including the side upper and side lower. Given the above restrictions, the probability that you wont be able to fit it below the lower berth of the adjacent passenger is low, however you cannot neglect it.
There are no hard and fast rules regarding who places luggage where. Rather, luggage is generally "managed" amicably by the passengers themselves or the TTE (Travelling Ticket Examiner) intervenes, but in most cases it is not a problem.
