Can parallel downloading with many ip's possible?
If the limitation is coming from the site, then it depends if the server allow you to resume partial downloads. If yes you can have two devices, one downloads the first part of the file, and the second one downloads the second part (telling the server it has already the first part).

The IP-address is given to you by your internet provider and it is unique. So you only have one and it can't be changed with software. (unless sometimes when you restart your router you can have a different one, but you can only have one at a time).

The device which downloads the second part needs to be connected to a network with a different ip-adress (for example your neighbor's Wlan).

But i don't think this is very practical.