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?
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.
I had Raid 0 for a while, but noticed only a slight overall improvement. I was disappointed with the whole idea and got SDD for my system and program files, and my conclusion is that no other affordable computer upgrade will gain you so much performance, before I got SSD I really didn't realize how much of bottle neck my HDD was.
Back to the original topic, depending on how you would use you computer, it might be faster if you have two hard drives connected to different controllers on Motherboard, if it has more then one. Sata ports for the same controller usually marked with same color eg. my motherboard has 4 orange ports controlled by Intel chip and 2 red port controlled by J-Micron chip. Connecting both hard drives to the same controller will reduce performance, in my tests, writing speed of a file copy from one HDD to another dropped more then twice compare to setup when hard drives where connected to different controllers.
It can be useful to have two drives because you can take advantage of parallelisation. For example, you can copy a file from one drive to another drive much faster than copying a file to another location on the same drive.
This parallelisation is also what improves read and write performance with RAID 0, and read performance with RAID 1.
Two drives will be faster if both drives are often used simultaneously. Eg, in Windows you can page off of one drive and put your files on the other, or put system files on one and user files on the other. Depending on your usage patterns this may cause both drives to be busy simultaneously a reasonable amount of the time.
However, two drives won't significantly improve performance (but won't do any harm) if you simply put a random chunk of your file system on the second drive -- it takes some conscious planning.
(The two exception to this are IBM System i (or whatever it's called these days) and some RAID systems. System i spreads (almost) all data out over all drives more or less uniformly, splitting single files across multiple drives, so that drives tend to be equally busy. RAID systems similarly spread individual files across multiple drives, but whether this improves performance (or instead makes it worse) depends on the particular RAID scheme.)
It used to be that average drive access speed got worse with larger drives, providing another motivation to use multiple smaller drives vs one large one. I'm not sure that this is so true anymore, though. (Haven't looked at drive performance specs recently.)