They are connected with the plate in centre that bearings fasten too and they would obviously be fastened at the ends into gantry sides. This was a quick mock up for you to visualise and obviously you'd strengthen has seemed appropriate.

To me there's no point wasting time drawing anything if your not working to dimensions you know are available. Those 2 tubes are standard dimensions for off the shelf steel 100 x 3.2mm. Rest like angle and flat plate is again off the shelf.
I've not done any bending or testing calcs but I'm sure combined strength of 2 tubes will be better than one large tube unless it's wall thickness is much greater.?