DVI, or Digital Visual Interface standard was created to replace the legacy analog VGA standard of several years ago. It has helped move the display world into the digital age, and been a big part of increasing the quality of output to our display devices. The problem with this transition is that you inevitably run into compatibility issues with older monitors that still require an analog signal for display. And while you can split DVI signal with a DVI splitter, you cannot simultaneously send both an analog signal and a digital signal.
Although recognized as a digital cable standard, the DVI cable actually comes in three distinct varieties: DVI-D, DVI-A, and DVI-I. These stand for Digital, Analog, and Integrated respectively. The first two types are apparent in their titles, but the integrated takes a bit of explanation. Integrated refers the cable being able to handle both Analog and Digital signals, depending on what is being output from the system. Again, this is either/or and is not able to send the two signal types simultaneously.
In order to split the signal to an analog VGA screen, the DVI output must be in either DVI-A or DVI-I. In most cases it makes more sense to purchase the I cable as it provides greater flexibility and allows for future upgrades to digital. Finding a DVI I to VGA Splitter should be too difficult, and are available at many local shops and online stores.
Although splitting the signal makes sense in theory, in practice it can be a different story. A number of other variables play into whether or not you are going to have success with your DVI splitter cable. It is also important to understand that splitting a signal is only able to provide mirrored images on the two monitors, and will not provide an extended screen experience to the user.