New GST Registration for two entity

I would like to know if Company "A"  into Information Technology gets registered with GST parallelly starts online e-commerce portal "B" in incubation stage as aggregator between buyers and sellers  .
Can A show B as sister firm and use same GST and bank account details also mentioning on e-commerce as sister firm of A or need to do it separately.
Does we need to file return when as per norm TDS @ 1% .

Also what would be the best practice you would suggest.