If your computer is outdated can you just plug 2 older computers together for twice the power?

Yes, and no..

If you are doing large quantities of similar calculations, as supercomputers are often uses for, or such as video rendering where the individual frames can be created on different computers then sequenced together, then a "cluster" system can use many low power computers to give high overall processing power.

But, the user interface and graphics etc. are still limited to whatever machine they are running on.

So for normal programs that run on a single machine, you are stuck with that machine's speed and capabilities.

Look up "clustering" or "Beowulf cluster" for more info.

No. 'cos life's a b i t c h.

unfortunately, this would produce the same results as having 2 B students working on a quiz together. it would NOT result in an A grade.

you cannot increase speed by using 2 slower CPUs. it just doesn't work that way. if you look at the CPUs from just 10 years ago, and the latest ones in production, they are not only smaller, but somehow have MORE PINS, meaning they can process that much more information at once.

my first computer had 237 pins. this was in 1999.

my current computer has 1150 pins. that is over 4 times as many, and the processor itself is about 2,600x faster than my first CPU.

newer computers' CPUs are faster than THAT.

so simply adding another computer won't be the big boost you think it will. you will just end up doibling your electric bill, when you COULD have just used that money to buy a fadter more efficient PC...

You're referring to a computer array. What you're describing is possible, but probably wouldn't work with just two old computers.

no, it doesn't work that way, a computer will be slow because it has a processor that isn't up to snuff to be able to handle the amount of data that needs to be handled in modern software, stringing two old computers together will not fix this, since only one processor will be used to process this data, another thing that makes a computer slow is the speed and amount of the RAM being used, RAM stands for Random Access Memory, and it isn't used for storage, it is used to speed up the process that your computer uses to access information, as computers have progressed they have created faster and faster RAM, the speed of the RAM being used is dictated by the motherboard, or mainboard, of the computer, so you wouldn't be able to just install faster RAM, however there may be something you could do to improve the speed by combining the RAM of the two computers, if both computers use the same speed of RAM and they are not already maxed out you may be able to remove the RAM from one computer and install it into the other boosting your RAM and speeding up your computer, Depending on your computers this may not be able to be done even if they are the same speed of RAM, often RAM needs to be installed in equal amounts, so if you have one computer with 1 4GB RAM chip and the other has 1 2GB RAM chip this would not be able to be done, you would also have to have open slots on the motherboard for RAM, even if the RAM in the two computers isn't compatible you may want to look at getting RAM for the faster of the two and see how it works, RAM now a days is cheap, Increasing RAM is one of the easiest ways to improve the power of an old machine, you may also want to format your hard drive and do a full reinstall of the operating system, this can really speed things up, but make sure to back everything up before you do this 

How wonderful it would be if you could take two old computers and just plug them together to make one fast one. Yep it doesn't work that way it's like two rotten apples you can't grind them up and make a bowl of good applesauce