The only reason why you could possibly be interested in using two Arduinos is that you refuse to do a proper job using one and, in the unlikely event that you successfully bring a Uno into the game, the best memory increase you can expect is about 10%, which was probably never worth the effort. I rather suspect that, if you ever run out of memory with a Mega, you probably need to be using something other than an Arduino.
Let me explain in more detail. I have plenty of memory left of the mega, I am just short on the sram memory. I've used, the F()macro as much as I can, but when it comes to using the aruino as a server, using the client.print(F()); results in everything not being printed because it takes to long to pull from whatever memory bank F uses. As a result, if you wanted a full page to show up, it stops at half page. So you can't use the F() macro, the same appears to apply for writing to an sd card.
So I was thinking trying it out with the uno, because I have an extra laying around, and if I can get a simple one boolean command to work i'll purchase the due, which I am confident will be more than adequate, So I can use the mega for the shields, and boolean calculations, then send the boolean results to the due, which the due will then work as the server. If I can get the due to work with a wi-fi shield that is. The due should add an additional noticeable amount of memory. Of course, I have no idea how to go by making them talk to each other to share the load.
What I really need is a super computer server, like the one Nick Gammon uses to host his video game. My resources are limited though, and I'd like to figure out how to do everything on an arduino before trying to learn how to program a sever though. With any luck i'll try to get my hands on one in the next two years.
I wonder if a Raspberry PI (or a cheap laptop) would be a better companion for your Mega? Just use the Mega for the stuff an Arduino is good at.
I've looked into getting a raspberry pi a little. I'm still getting use to arduino technology and the pi appears to be much more complicated. Do you ever use the pi? I'm curious how I would go by using a laptop to share the load with the arduino? That would be all the memory and processing power I would need.
I must say I have no experience with it, but you can... apparently add ram to an mega2560 using a quadram - shield/module
It should give you 512KB of RAM and a smaller one, mega-ram with 128KB is available as well.
This seems like a possible fix as well. Has anyone used a shield like this before?