7000 Series Ermeto Steel Flareless Fitting, 0.25 Inch Tube OD Union Tee

Steel and stainless steel both work fine with this fitting, and you can also run it with aluminum, hard copper, or half copper tubing. The one thing to keep in mind is that it's built for extra heavy wall tubing specifically, meaning standard thin-wall stuff that can be flared is not the right match here. If your stainless and steel lines are the appropriate wall thickness, you're in good shape to tie them together at a tee junction.