There are a number of reasons that you may want to attempt to split your DVI signal from a computer. The most common circumstance is one in which you would like to simultaneously transmit a duplicate image of one screen onto another screen via projector, LCD, etc. And while conceptually this can work, there are a number of criteria that must be met before a DVI splitter will work in this scenario.
A DVI splitter is designed to send a single output signal to two separate devices. For this to work however, the first requirement is that the receiving devices must be able to accept the same type of signal. With this type of simple signal you simply cannot send a signal to both a digital device as well as an analog device.
It is generally also helpful for the two screens to be of similar size and specifications, particularly contrast ratios. Again, basic DVI splitter cables will only send one signal, including size, ratio, color, frequency, etc. If your two devices require different signals, one of the other will have a difficult time converting.
A common misconception about these types of cables is that they are able to “extend” the screen size onto two monitors. This is simply not the case. A splitter cable is generally only able to send duplicate or mirror images to the two screens. You’ll need a different output device entirely to extend your desktop.
The best way to save yourself the trouble of figuring out how to properly use a splitter cable is by bypassing it altogether. A more effective and consistent way to split your signal is with a DVI converter box. These converter boxes will more or less take care of all the conversions that may be necessary for your monitors. You don’t have to worry about analog vs. digital, and you typically don’t have to worry about signal degradation. Expect to pay a bit more for the converter box, but the saved time and effort will be well worth the expense.