In Terminal that is possible by using apt-fast (using Axel).
But is it possible to put to work Axel and apt-fast in Synaptic Manager?
I found this suggestion, but I would not attempt it without the assistance from Ask Ubuntu :)
The method in the forum thread (tricking Synaptic by renaming
Personally, I would do what LnxSlck said in his comment, set up a VM with Ubuntu and test there. That would allow you to test without any real issue. If it breaks, don't do it. If it works, you can be semi-safe in the idea that it will work correctly without any true incident.
I tend to do this with every "test" i do with Ubuntu, so its a good practice if you dont want to explode your installation by accident.