I have encountered some kind of problem with minecarts that is affecting a project I am doing. So I am trying to make a thing made out of two minecarts, which is easy but the problem is making it move and then when the players try to interact with it.

/summon minecraft:minecart ~ ~1 ~ {CustomName:"{\"text\":\"FlyingMachine\"}",CustomDisplayTile:1,DisplayState:{Name:"minecraft:stone_slab"},Passengers:[{id:"minecraft:minecart",CustomName:"{\"text\":\"mcsteering\"}",CustomDisplayTile:1,DisplayState:{Name:"minecraft:oak_fence_gate"},Invulnerable:1,DisplayOffset:5}]}

Is the entity I am using currently. (The bottom one is invulnerable because I need them to only break the top one)
As for where the glitch happens, when using this command:

/data merge entity @e[type=minecraft:minecart,distance=..10,limit=1] {Motion:[1.0,1.0,0.0]}

It then makes the minecarts unclickable in that it makes the bottom minecart's hitbox completely seem to encapsulate the bottom minecart's hitbox. In 1.8.3 it also seems that to make this happen, you need to use the command twice, the second time while it is in the air, for it to create this glitch.

The first image is the minecart before, which is possible to get into, but the second image is after executing the second command twice, the second time while the minecart was in the air. The minecart looks the same, but seems to not be rideable anymore, and cannot be right-clicked.

EDIT: the comments below contain more info as to the current problem, but the new main problem is that using the command:

/summon minecraft:minecart ~ ~1 ~ {Invulnerable:1,CustomDisplayTile:1,DisplayTile:fence_gate,DisplayOffset:5,Passengers:[{id:"minecart",CustomDisplayTile:1,DisplayTile:stone_slab}]}

produces an unrideable minecart.