Wet room floor waste connections

We had an ground extension "bedroom and en suite" built but due to building cost increasing we decided not to have the en suite fitted and opted to do this at a later date. it has now come to that time where we are deciding how to get this fitted. which is where we feel we have a problem, There are two drainage pipes each side of the room, one for the toilet and one for the "walk in shower section". (these pipes are the same type, size etc and are both capped off currently) i know from watching the builders that both these pipes join under the extension and lead to and drop down into the manhole cover at the rear of the garden. on looking into shower drains options it seems this above mentioned connection is not correct, and the shower drain cannot drop directly into essentially the same sewage pipe because once we open the seal cap to install the shower grate this direct access will lead to the sewage gas coming up through the shower drain into the bathroom. so my question is: is there a method i.e some kind of connector to just add to the pipe to allow the shower waste to go straight down this drain (i.e a vertical waste that just slots on the end of this pvc waste pipe) or are we going to have to forget all about the shower drain and install some other piping method.