Question: Assuming that the drives all have the same RPM and overall same specs except for storage space, would it be faster to install two hard drives together or just one larger one in a computer?
Answer: Yes. Two drives are faster.
As someone already said, you can use RAID 0. This will give you almost double the speed for anything larger than a small file, where small depend on how your raid is formatted. This makes both drives appear as one large, so there is no extra steps to get improved performance once the RAID is setup. The one disadvantage of RAID 0 is that if ONE drive fails, you lose ALL your data.
You can also simply use both drives separately. In this case the performance advantage will depend how they are used. To get maximum advantage you will need to ensure both drives get used simultaneously. If you run two or more disk I/O bound applications, that is relatively easy. If not, you can separate the OS, Swap (if used), applications or data. What to do really depends on the types of applications you use.
Note, do not confuse drive and partitions. Even though Windows presents them as different drives, using two partitions can only slow things down because when they are used simultaneously, the head has to move from one partition to the other since they are separated by cylinders. This does not apply to SSDs though.