Usually the runner knows which race is the most important for him this season – it is his main objective. But sometimes you may have planned 2 or 3 races that are all equally important.
If you realize it’s a prep race for another objective, add it as a prep race.
If there are 3 months or more between the two races, you can set the first race as the main objective. You can then enter the second race as the new main objective. This will give you enough time to prepare for the 2 races.
If you are 1 or 2 months apart and the runs are very close in distance (e.g. 2 runs of 50-60 km), you can enter the first run as your main objective and then, after you have completed the first run, enter the second as your new main objective. Or you can set the first run as an intermediate objective and the second as the main objective. In both cases, the training will be very similar.
If you are less than two months apart and your objectives are very different, or if you have a road and a trail objective (e.g. a road marathon and a mountain ultra trail), you need to decide which is more important to you. It’s utopian to think you’re 100% ready for both objectives, so choose the objective that means the most to you and make it your main objective. The endurance you have developed for the first race will definitely come in handy for the second race.