If time permits, it is very helpful for both players to learn both parts of the duet. This may sound like a lot of work, but it will be a time saver in the end. Being completely familiar with where the difficulties are will allow you to be prepared for speed bumps and show a little mercy to your duo partner when and if necessary.
Be just as prepared to listen to both parts as you are to play them. This may seem hard at first, but it is a skill worth developing and becomes one of the joys of playing with others.
Lagartija's suggestion regarding visual cues is invaluable. Orchestral musicians learn to keep an eye on the conductor, even while they are reading their own music. Similarly, full awareness of what your partner is signaling will help you to stay in synch.
A few planned ritardandos, can be added if appropriate. This gives you an opportunity to catch your breath (sometimes literally) and to establish a more solid tempo if one or both of you have started to play like a runaway train...perhaps due to nerves (or just to get it over with).
Last suggestion is pretty obvious, but I will include it anyway. Rehearse beginning together and ending together no matter what happens in between.
Try, above all, to have fun. It will be well worth all the effort you are putting into this endeavor. Good Luck!