As a voice actor, you may sometimes be asked to do more than one role in an audio production, possibly a leading or supporting role plus one or more extras. Producers often ask for this in order to minimize the number of voice actors they need to work with. It is very important that each character sound different, so that the audience can tell them apart; so, even though voice acting is more about acting than voice, it is important to know how to adjust your voice.
Approaching the Problem
Obviously, there is wide range of voices possible; however, aside from certain gifted mimics, most people cannot simply imitate other people's voices. Fortunately, that is not necessary. You can apply a few techniques to create voices that do not sound the same.
Varying pitch
Probably the simplest way to distinguish two voices is to use the top half of your range for one and the bottom half for the other, as in this example: high voice vs low voice example.
It's not hard to figure out that both roles are done by the same person, but that doesn't matter. What matters is that you can tell when each character is speaking. Of course, you have to make sure that the "low" character never speaks with a high pitch and vice-versa, but this is a technique that is very easy to learn.
Varying texture
You can further distinguish voices if you use different textures: you can use a gravelly sound, a nasal sound, a throaty sound - there are many possibilities. Combining this with the different pitches can be very effective: texture example.
Accents
Yet another possibility is to use an accent. Obviously, accents can be used as needed for characters in general, but using differing accents can also help distinguish characters, even if their voice types are otherwise close: excerpt from a '3 characters' scene.
Other variations
The above are just a few examples of ways to distinguish voices. You can also vary personality, pacing, and so on. The important thing is to be consistent within each voice and to use non-overlapping sets of characteristics. As a rule, if you have a scene with more than one of your own characters, record each one separately and then mix them. It is very difficult to maintain consistency while switching back and forth in a single recording session.
Practice
Voice practice can involve saying actually anything at all. Try recording yourself reading a paragraph in a book, or a script. Do multiple recordings and play them back. See if the voices are really similar when you want them to be and different when you want them to be. It can take a while to develop a voice to the point where you really like it.