|You can't split a network cable like that.|
Is there a device that would permit me to plug the cat 5 cable in to it with two ports from which 2 additional cat 5 cables could go to each of my computers?
Yes there is and it's called a "switch". They connect easily and require no setup on your part.
if so, could the i-net be accessed simultaneously with both computers at the same time with such a device?
Yes, with the switch plugged into your router, all devices plugged into the switch would be able to access the internet at the same time.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.