Hi
If you look on Peter Fogwill's Aquatec site he has a trailer that links 3 of these tanks together. It is not very clear but they are fitted into a trailer frame with 2 steel u shaped brackets that hold the tanks together and prevents them moving sideways. A piece of angle iron is bolted to the trailer frame at the front and back to prevent the tanks moving either forward or backwards. The width of the u brackets is exactly the width of the 3 tanks together.
You could have a frame fabricated from angle iron welded up with two securing u shaped steel straps welded onto it. The width of the u shaped brackets and frame would be the same as your 2 tanks together. The frame would then be slipped over the top of the 2 tanks, pulled down and secured your van floor. This frame will hold the tanks snuggly together.
The 125l tanks are not baffled and you will experience a lot of rocking of your van with half tanks by the water moving around.
The 3 trailer tanks were linked together with stiff walled hose to prevent it collapsing flat by the pumps. There are no on/off valves and this allows the water to find its own level between the tanks which is a good thing if the tanks are fitted length ways. The issue with this is that if your van is on a slope with low water in the tanks, the water will drain to the lower tank and leave the upper tank outlet to suck air.
Hope you can understand this.
Spruce
ps. just remembered that I had a picture of my trailer before I scrapped it.