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I have a 72"x18" tank that will have a depth of 12-24". My filtration will be a pair of Filstar XP3's (350 GPH) with inline heaters. The inhabitants of the tank will be whatever L-#'s I can get my hands on in Central Illinois (panaque and ansistrus sp. are my top choices). I will fill the tank with the wood seen here.

I am going to be using 12" spraybars, and I think my choices are:

Both bars on one side with intakes on the opposite side of the tank. Variations to this include putting one bar high, and one low for a flow cross section to cover the tank and perhaps putting one intake in the center of the tank to make half of the tank lower flow.

One bar on each end, with the intakes in the middle.

Put the bars against the back of the tank, and then put the intakes towards the sides.

I would put the both spray bars in vertically not horizontally. I think your Idea of having one of the returns part way could work... but don't forget that the wood and any other thing in the tank will inhibit the flow. I Think I would just put them on opposite ends and lose one of the inline heaters.

There are several ways to get good circulation in a tank, and Whitepine's ideas are all good - except I quite like mounting the spray-bar horizontally along the surface - not because it's very "natural", but because it gives the best surface movement.

I have three different pumps in my 100g/400l 5ft/150cm tank. One is just a small Eheim pump, which does nothing but pump water around in the tank, the other two are filter pumps, Eheim 2028 and the Juwel Jumbo that came with the tank. All pumps are rated at 1000 l/h, which means that my 400l is circulated around 7-8 times per hour.

Having multiple sources of water flow allows you to control the flow of the water more than having just one source, and there's no firm and fast rule that you must do it in any particular way. Even though we often see rivers as one flowing volume of water, if you look carefully on a river, you'll find that there are all sorts of current flowing in all sorts of directions, with still sections and flowing sections, some parts that form eddies, etc, etc.

So whilst it's nice to aim for one linear flow from one end to another, that's not necessarily how the flow goes in a river...

Sorry if this is more ramblings than particularly useful advice...

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Mats