I have seen the 1,000' number a few times for how far you can pump air without any "losses". Here's my question... If I run two 500' runs from my manifold am I at that 1,000' number or no? Could I run two 1,000' runs from my manifold? Is this number any different for a windmill vs an electric compressor?
How far you can go is a function of your pressure loss mostly. The longer the run, the more cfm you pump and the smaller diameter tube, your pressure loss goes up. One can calculate pretty precisely how many psi you'll lose if you know all the variables. Losing 1-3 psi is pretty doable. If you're losing 10 psi, your compressor is working harder and may run out of oomf.
There will always be loss for the run.
If you do two runs, assuming they are feeding similar pressure loss diffuser systems, it will split the cfm between them. So your velocity is lower and you need less diameter. Essentially, I'd do the pressure loss calc for each run, but for a nominally balanced pair, they are each a 500' run and losses do not add.
It all comes down to the velocity of the air. Keep it down and you can go a long way, but you'll have to go bigger tubing and the cost will eventually get you.
Here's a formula for you.
http://www.tlv.com/global/TI/calculator/air-pressure-loss-through-piping.html 1 bar is 14.5038 psi
DN15 = 1/2: ID
DN20 = 3/4" ID
DN25 = 1" ID