Here are some thoughts to consider:
1) Best results are obtained if you train under a qualified sprint coach.
2) You must “train” for your sprints (just like you would any other sports/fitness activity).
3) By “train” you should have an overall program, to monitor and to progress your performance, with periodic reviewal and tweaking of your program as needed. This will be best done by a qualified coach.
4) So, whether you train once a day or twice a day, and exactly what you will train during each of those sessions, will depend on the overall training program.
5) Overall training program will depend on many factors…periodization, upcoming competition schedules and such.
6) You also need to think about a VERY IMPORTANT FACTOR, which is Recovery. You don’t get stronger/faster etc., in the gym (or right there on the track). However, when you train (in other words, when you provide an appropriate physical stress to your body), your body then recovers (adapts to the stress provided) and you get stronger/faster etc., depending upon the nature of the appropriate stress that was provided.
7) If you provide too much physical stress (sprint too many times a day, or sprint to your maximum capacity all the time you are on the track, or lift too much weights too soon etc.), then, you are not following a training program. You are just exercising… This will not lead to progress but, will likely lead to injuries.
8) Even if you are a sprint athlete, there will be times when your personalized training program calls for high intensity (maximum or close to maximum sprints or a percentage of your maximum sprint efforts) and there will be times, when volume (how long you run) at lower intensities are called for.
9) Recovery depends on two things —-> Rest and Nutrition.
10) You should rest adequately to allow your body to recover well. If you train too soon, before your body recovers fully, over time you will get into an overtraining zone and this is NOT good. Not good for your performance, which will noticeably regress instead of progressing, and not good for your overall health either, (both physical health AND mental health).
11) If you get too deep into the overtraining zone, it will likely take a LONGGG time for you to recover and to resume “normal” training again.
12) Same thing with Nutrition. You MUST eat adequately, and you must eat appropriately. Enough carbohydrates, PROTEIN and fats are all required for proper recovery.
13) Ensure that you are taking in/making adequate amounts of micro-nutrients (vitamins and minerals). Periodic testing at reputable labs is required to ensure that you are maintaining proper levels of all micro-nutrients.
14) A good / qualified coach should/will ensure that all the above are taken into consideration, when drawing up training programs. This is the exact reason, why it is BEST to train under a qualified coach.