If both sides agree, can they both use one lawyer?

That is a loaded question! If the “sides” are married to each other, the matter is not a legal separation, divorce or dissolution issue, and they are doing a lease of their land to some third party, as long as their interests are aligned, it is perfectly appropriate for them to use the same lawyer. The tenant, however, will need to use a different lawyer if he or she uses a lawyer at all. In court cases and in negotiations where parties really are in relatively opposite or dissimilar situations, it is rarely if ever appropriate for them to use the same lawyer. For example, often people will want to do a dissolution of their marriage and seek to use the same attorney because they feel they have an agreement. That, however, is not appropriate. Can you imagine one of the parties going in to the lawyer in private and saying “How little child support can I get away with paying,” and then the other party going in the following day saying “What is the most child support I can receive?” It is an impossible position for an attorney to be in, so, while there is generally no requirement that both parties have to have lawyers, it is nevertheless the situation that in most circumstances it is not possible for opposing parties to have the same lawyer. In many dissolution of marriage cases, for example, one of the parties will hire a lawyer and the other one will represent himself or herself. Generally, that is not advisable because both parties ought to have lawyers, but where only one lawyer is used because the other party refused to hire a lawyer, this is a different situation from a case where one lawyer would represent both parties.
These things are often difficult to sort through, and we are generally available to talk with our clients about representing them, which parties are appropriate for representation by whom, etc.