[AuRavelling79]

Operation "rainwater harvest" is starting to take shape (except I have had to completely move my intended area and tank shape due to a hidden gas main!) and I will have a pair of tanks next to each other but about 400mm apart.

I want to join them together so the water is shared so at what height on the tank sides should I connect and how should I do it?

[Tom White]

I've connected water butts up together before, and I connected them at the top.

Why? 

Well, if you connect them at the bottom, you're asking for leaks; especially with the weight of the water pressure above the connection.

I reckon a standard water butt connection from a garden centre will do the job; you'll also need a drill circular thingy to cut the hole in the tanks.

If anyone can add, please do, this is just my initial thought from water butts and not IBCs.

[Rogue Trader]

i did this recently and googled ibc connectors and got sent a length of 2 inch hose some serious connectors to attach to the screw thread of the main outlet. I was shocked at cost , was about £65 but worked a treat

[H S and Son]

Here you go Malc http://www.coralculture.co.uk/shop/index.php?cPath=157&osCsid=ai1u571eo87rvpg422j4mv0mh3

good quality pipework and you wont beat Chris's prices.

Connect them at the top (about a foot from the top anyway) as there will be less water pressure at the top just in case the seals want to pop.

[Seymour Sunshine]

It depends. What do you want?

If you want to have the same amount of water in each and draw from each equally, join at the bottom.

If you want to fill one and then the other, join them at the top. Then you can draw from one at a time. This is how I have mine joined.

[spongebob]

As seymour says but it depends how you fill them. If your source/downpipe is only filling into one tank then you will want the transfer pipe near the bottom otherwise the 2nd will only fill when the first has reached an overflow situation. If you are only using 22mm then the tank is likely to flow over the rim faster than it can flow through the high level transfer pipe and thus waste water.
If you want any advice or the odd bits I've got loads of plumbing bits you can have. Just off j14 of the m5.
Andy

[Tom White]

There's probably a problem with getting access to the inside of the bottom of the IBCs, unless you're super skinny.

I reckon you're stuck at fixing them near the top.