It depends.
Sometimes people request that two law firms team up or collaborate to represent them in their divorce case. This might be because one attorney specializes in a particular issue or area of law that is relevant to the case. For example, there may be a real estate or business attorney representing the interests of the business or business owner involved in a divorce.
Generally, if the attorneys agree on how to share the fees and work load then it’s not an issue. Sometimes, in high conflict litigation that is likely to be appealed, the law firm likely to handle the appeal (if different than the firm representing the client at the trial court level) may be retained earlier in the litigation stages to avoid conflicting out later, or to get a better handle on the proceedings from the start.
So, if you are willing to pay for two attorneys to represent you and you believe that your interests are best served by having more than one attorney represent you, it is permissible to have more than one attorney representing you on the same divorce issue.
On a related note, larger firms with multiple family law attorneys will sometimes split up case work among several lawyers. In this scenario, you could have more than one attorney working on your case without even knowing it. If you are uncomfortable with this practice, ask your lawyer about his or her individual policy regarding delegating work.