This is a difficult question to answer, and there is little scientific data to guide recommendations.  It is very convenient to have all vaccinations given all at once to save time and money, and it is very commonly done with little adverse effects.  There are no studies examining the safety or efficacy of the vaccines when multiple different ones are administered together.  This does not mean they are not safe or not effective if given in combination, just that this has not been proven.  It does appear that there is a higher rate of adverse reactions if multiple vaccinations are given at one time, however, the overall rate of adverse reactions is still very low.  If your horse has experienced an adverse reaction in the past, it may be wise to split up his or her future vaccinations, both to decrease the risk of an adverse event as well as to determine if a particular vaccination is the culprit causing the horse to react.  If multiple vaccinations are given on the same date, it must be considered that modified live vaccinations should not be given close to the site of a killed vaccination, as the adjuvant in the killed vaccine may interfere with the activity of the modified live vaccine.  Seek your veterinarian’s advice if this is a concern.