How to set multihoming for download from multiple Wi-Fi connections?
No, it is not immediately possible.

If you had a server that you control in between you and the server you want to download from, it is theoretically possible.

You would install several wireless network cards (or USB WiFi dongles or whatever) in your computer. Setup these up with different IP-addresses.

The server you control (outside your wireless network - i.e. on the Internet somewhere) would download the file for you, edit the received packets to load-balance between your IP-addresses/ports for the wireless interface you have, and send them on to you.

On your computer you would need a special program to receive those packets and reassemble it as a single TCP stream and pass it on to the actual program doing the downloading.

This could more generally be achieved using some sort of VPN software.

However I don't think anything readily available for Mac OS X exists that has this functionality, so you'll have to build it yourself.

All in all, unless you are very interested in networks and computers, it is not easily  achievable.