Hi Aly,
Design criteria should be based on the RSSI and not necessarily on the EIRP. For instance, you can have a criteria like this:
90% of the coverage area should be covered by -85dBm RSSI.
This means that each indoor antenna can have different EIRP, depending how big the area to be covered. One antenna can have 2dBm because it only covers a small room. while others especially if located at the hall way, can have 8dBm because it has to cover more areas with concrete or wood partition.
Using the Free Space Loss formula (FSL) together with some losses like the Body Loss and Partition losses you can have a reference value of RSSI on each antenna. FSL formula = 32.44+20*LOG(Distance to cover in km)+20*LOG(Frequency in MHz.
You can have the same value of EIRP for each Antenna if you have the same target coverage area per antenna. There are components used in DAS for you to balance the power, you can put more power to one antenna and less on the other. These components are Power splitters/Power divider and directional couplers.