tie together

I am in the process of adding a washer and dryer on the second floor of my two story home. They will be located on the other side of the wall from a bathroom group (tub/shower, toilet, and lavatory) so I have relatively good access for routing the drains and supply lines. My question is what is...

(short lead in... had an addition build a couple years back and a Rheem ECO200 9.5 GPM tankless unit installed - addition is 1 bth, kitchen and laundry - with the anticipation of tying it into the main section at some point) Question: what is the best practice to do so? Would it be best to run...