It is best not to accept more than one offer to avoid burning your bridges for the future. If you wish to delay acceptance of an offer, it is important to express first your appreciation for having received the offer and your strong interest in the position. Next, explain that you are in the process of weighing several offers, and that you want to be certain you make the right choice — for both yourself and the employer. You could suggest a time frame you have given yourself to make a final decision (keep in mind, only a few days to a week or so may be considered reasonable), or you could ask the employer in what time frame he or she needs your decision — or both.
Remember, the interviewer is human, and typically rejection is not among our favorite experiences, i.e., an employer does not want someone who doesn't want them. You need to make the focus about this being an excellent opportunity and that you just want to be sure you make the right decision for both of you.