This is a collection of stacked freeway assets.
They're free placeable.

Imgur album:
https://imgur.com/a/E9dYN#0

The GD stacked freeway assets are 'Intersection' assets, that you can place in game and also in the map editor (only the highway variants).
The 6-lane one way road variants are only accessible in game.

The orientation and asset names are specified for right hand drive maps (RHD).
On left hand drive maps (LHD) everything is inverted.

Before you place an item into the game or the map editor you have to toggle the 'NoPillars' mod to 'Floating'.
Afterwards you can toggle it to 'Pillars' again.

They're done with default highway (or 6-lane one way road for the 6Fwy variants), bottom level going in one direction, top level going in opposite direction.
Because actually I didn't find a workaround to make assets like theses snapable to each other, you have to place one, attach a piece of highway (or 6-lane one way road for the 6Fwy variants) at top level that is exactly 3 units (24m) long, then attach the next asset.
If 3 units is not possible without 'Road Anarchy' then try it with the view changed to top-down.
Afterwards you can easily connect the bottom level of the both assets. Toggle the direction with the road upgrade tool by right-click if necessary.

You can also just place the assets next to each other by eyeball and connect them afterwards with highway segments (or 6-lane one way road for the 6Fwy variants).

You need the mod 'Precision Engineering' by Simie -> http://steamcommunity.com/sharedfiles/filedetails/?id=445589127

There is no grid to snap to inside the map editor, so you should use the 'Configurable slope limiter' mod and hold SHIFT while dragging the highway connectors, very smoothly, pixel by pixel (yes this sucks), as soon the shown number for the units/meters switches from 3u / 23m to 3u / 24m it's the right position.
If 3 units is not possible without 'Road Anarchy' then try it with the view changed to top-down.
Now place the next asset, then draw the missing connector for the bottom level.

Don't forget to change the direction of the connector elements if they point to the wrong direction.
Just choose the road upgrade tool and right-click on that highway segment.

Inside the game it's way easier:
Just place one asset, attach a 3 units long 2-lane one way road (!) on top level and attach the next asset and so on.
The same here: If 3 units is not possible without 'Road Anarchy' then try it with the view changed to top-down.
Then you can upgrade the one way roads to highways and correct the direction of the connectors with the road upgrade tool by right-click afterwards.

On both ends of the connector are nodes where you can connect ramps.

The street lamp posts of the bottom level stick inside the concrete of the top level, but they don't come through.
As long as there are no custom roads without these lamp posts which is also available in the map editor it's just like that.

*You need the mod 'NoPillars' by some danny:
http://steamcommunity.com/sharedfiles/filedetails/?id=409073164
or
NoPillars ( v1.1 & European Buildings Unlocker compatible ) by BloodyPenguin
http://steamcommunity.com/sharedfiles/filedetails/?id=463845891

You also need the necessary props:
'gate pillar prop [1-eleveated 3tile width]' by duck clogs -> http://steamcommunity.com/sharedfiles/filedetails/?id=448910789
'gate pillar prop [1-elevated 4tile width]' by duck clogs -> http://steamcommunity.com/sharedfiles/filedetails/?id=448912201
'side pillar prop [1-elevated 2tile width]' by duck clogs -> http://steamcommunity.com/sharedfiles/filedetails/?id=448910008

If you can't place the asset you may flatten that area with the 'Terraform Tool' by rollo:
http://steamcommunity.com/sharedfiles/filedetails/?id=411095553

If that doesn't work for you or you don't want to flatten the area, use the mod 'Configurable slope limiter' by b.fokke:
http://steamcommunity.com/sharedfiles/filedetails/?id=440635326
Press Left-Shift while trying to place the asset and it should be placeable.

If that also doesn't work for you try it with
'RoadAnarchy' by emf:
http://steamcommunity.com/sharedfiles/filedetails/?id=418556522
or 'AdvancedRoadAnarchy' by northfacts:
http://steamcommunity.com/sharedfiles/filedetails/?id=433567230
or the new fork of 'Road Anarchy' that also replaces 'Fine Road Heights' by japa
-> 'Enhanced Road Heights' by WeWill GrokU:
http://steamcommunity.com/sharedfiles/filedetails/?id=448434637

'RoadAnarchy' and 'Enhanced Road Heights' are toggled on by default and you can toggle them off/on in game by pressing 'L'.
'AdvancedRoadAnarchy' is toggled off by default and you can toggle it on/off by pressing ALT + 'L' and you get a visual feedback.

I recommend 'AdvancedRoadAnarchy'.