make the HO a dog bone, then you can run continuously, or if you have the O-scale drop under the HO, then put a 3rd level up, one above the HO.. then put an helix at each end for HO. or......... if the O drops under the HO then you can also run the HO off the end of the HO layout and drop the HO under the O scale . to take the HO all around the rest of the room kinda hidden. that way if you have a double mainline in HO one line the train will alway go across the ho layout from left to right, and the other line from right to left. so you never see the same train go across the layout in both directions, make it so that when the HO leaves the layout that its going to an imagery far off place, and it can come back on from the other side from an imagery place.