Can you hook up two dc into one system. For instance, putting two 3hp dc at the end of the main line of the pipe. I would assume that the chips and dust might collect more into one of the machines but would it help with cfm and air flow (assuming big enough piping)

The piping has to be sized correctly to give you the correct velocity. If the duct is too big velocity drops and the dust will fall out and settle at the bottom of the pipe. Not enough air (pipe to small) and the impeller will free wheel, it has nothing to bite against.

Can what you are suggesting work? Probably, if done correctly. There is very little upside and twice as much that can go wrong.

What Bill said. First, what are you trying to accomplish, solve a problem or additional functionality? To maintain 3500fps in a main you might use a 7"-8" based on a 1200 cfm collector, 2 systems approx. 2400 cfm would require 10"-12" main to utilize both systems. So back to the question, what are you trying to accomplish?

I'm using Dewalt DWE7480 since last some months, It is the best for dust collection, It comes with 2-2/1" dust port and the best part is you can extract with 2-2/1" port

You can add two dust collectors, either in serial or parallel at the end of the main line. If in serial (one feeds directly into the next one) the overall static pressure will be increased. If in parallel, total CFM will be increased accordingly.

There are a number of gotchas if you plan on doing this. So be aware. The efficiency of two collectors working together is always much less than a single collector with double the power rating. This due to fan size, ducting, and a bunch of other factors.

Second, if the blowers are unequal in size, say a 1 HP with a 3HP, and they are hooked in series, the 1 HP might not be loaded enough and burn out early. So blowers need to be the same size.

If you’re retrofitting, you need to consider increasing the ducting size or you’ll end up further decreasing the efficiency of the dual-blower configuration. Also you need to switch on and off two blowers instead of one.

I did it for my home shop, using two 2-hp blowers, but can’t really recommend it. The only reason to do this is cost (my motivation) because I got the blowers for cheap. Otherwise I wish I had bought and installed a single 5 HP unit.