Make sure you know the song that you are trying to perform. Nothing can be more embarrassing than getting up to sing a song, and not knowing the tune or the words. Listen to the song many times, to be sure it is in your range. Assuming that you have done so, practice listening to the song while reading through your set of lyrics with your partner. Speak them along with the song, making sure you know where to come in and leave off with each word.
Once you are certain that you and your partner know the exact pitch and rate the song will use, you may begin to warm up your voice. Many professionals suggest not drinking ice cold beverages, alcohol or dairy products before performing due to the limitations on the vocal chords. Drinking room temperature water seems to be the recommendation for quenching thirst. Begin by vocalizing the pronunciation of words that require a very high or very low tone, due to the difficulty of vowels in these areas. Do this as many times as necessary to have correct pronunciation of all words in both included vocal parts of the song. Remember that to achieve high or low notes, it is best to breathe from the diaphragm for the support to stretch the vocal chords to accommodate the note. Practice the highest and lowest notes and their corresponding words, to ensure the correct sound.
Once you have warmed up your voice, and practiced the placement of the lyrics, you should begin to sing along with the disk that has the vocals included, with your partner. Do this as many times as necessary to ensure both vocalists are hitting correct notes and coming in and out of the notes at the correct time. Nothing is worse than hearing someone perform karaoke, singing two beats behind the music. Timing is essential when singing with a partner. To ensure coming in together, you may want to use a hand gesture you are comfortable with to signal the note. Practice this many times, so that you are certain the vocals are together, rather than staggered.
Since we now have memorized the words to the song, try singing along with the karaoke disk, or the disk with no vocals included. Begin by counting off the beats until the first person to sing is supposed to come in. Many songs have a small musical introduction, and this will serve in helping you find where to come in. Begin by singing alone, without your partner, and have them listen to you practice your part. If there are any discrepancies, now is the time to fix them, while you can concentrate on your own part. When you are satisfied with how your part of the song sounds, trade places with your partner. The key to singing a duet is knowing each others voice, and knowing how your voices react together. Let your partner sing through his or her part as many times as needed to achieve the correct pitch, tone and placement of the notes.
Now that you have sang the song alone, and listened to your partner sing alone, it is time to sing together. Sing along with the karaoke version of your disk as many times as you need to, in order to have both voices coming in and ending at the same time. Make sure that you do not forget to stop the disk and go back if you have a problem with one coming in too soon or ending too late. Many duets practice for weeks in order to have the perfection they strive for. There is no prescribed time for practice, as each voice is different and may take more or less time to get achieve the range in use.
Once you have practiced your song with your partner enough to be comfortable with the disk, try it a Capella. If either of you have a problem with pitch, tone or rate, this will help you to quell it. Begin by tapping off your beat, and the first vocalist beginning their part of the song. If this is your first a Capella song, it may take you a few times to get used to having no music. But if you can sing your karaoke duet with no music, and it sounds the way you want it to, then singing with distractions in public will not be a problem for you. Have fun, and good luck with singing your karaoke duet.