My two cents..
We have examined some solution for new ATM system. One is to have APP room at present position.Below TWR cab,and another one to locate app position in ACC a few km away. Both solutions have advantages and disadvantages. The main disadvantage for separate solution is staffing.
In case it is "co-located" it is easier to organize shift at twr/app. Because everyone (or it will be aim) will be multi rated (TWR/APP) and people could work 2 hours on TWR than take rest,than 2hours in APP or any other combination that seems appropriate. Also we do not belive that it is possible to train people only for APP position without TWR experience . Thre are examples in different direction but we strongly believe that APP ATCO must have current TWR rating. Having such approach to problem there is no possibility for someone to become TWR OJTI withour APP rating. So each TWR OJTI must have APP rating . ESSAR 5(and local interpretation of it) clearly states that OJTI must have some nuber of hours at live traffic in order to be qualified to train students. With such requirements it is easier to mainatin required level of " live traffic work" in case of TWR/APP in the same building. It is also possible to do it when facilities are separated but life for SATCO will be much easier.
Co-location of TWR/APP costs more,because the main system is in ACC but few cables (maybe wireless) more is not huge costs. People and organization will cost more.
Thank you for your inputs in this thread it will help us.