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If you have a need for a 4-post rack, do not accomplish that by using 2 2-po= st racks. You will likely find that rack rails that are designed for a 4-pos= t rack will not fit.
Why? With *any* rack, there are always scenarios where the rack rails for some random item don't end up fitting right. That's certainly not a problem inherent to two 2-post racks. You can find 2-post racks in any number of interesting and unusual post/flange configurations. It's certainly true that picking any old random 2-post rack has certain hazards associated with it - the solution is don't pick "any old random" one, not "don't pick a 2-post rack." But the look-before-buying rule applies to any rack you buy, doesn't it?
The major issue is that 2-post rack rails are generally U-shaped, and have tapped holes. Server rack rails are L-shaped and generally have square holes. The vast majority of mounting rails I have seen in server equipment, in the last few years especially, will not fit because of the extra inside rails. Been there, done that, had to buy a real 4-post rack. Is it really a big deal to spend $500 for the proper rack? I do agree that some 4-post racks tend to be flimsier than the 2-post racks, but when properly assembled, any decent brand should work fine. Just make sure to double check the weight capacities. -Randy