This summary is not the rule. The rule text controls.
This rule used to cover situations where a lawyer acts as a go-between for two or more clients who have different interests, but who want the lawyer's help working out an agreement instead of fighting it out. A common example is a lawyer helping family members settle an estate or business partners work out how to split up a company.
If a lawyer takes on this kind of role, the rule required the lawyer to explain the situation clearly to everyone involved. Each client needed to understand that the lawyer was not representing just them, and that the lawyer could not give any one person the same full loyalty a normal client would get. Everyone involved had to agree to this arrangement before the lawyer could move forward.
The idea behind this rule was to let people save time and money by working through one lawyer instead of hiring separate attorneys for every disagreement. But it also protected people by making sure they understood what they were giving up by sharing a lawyer with someone whose interests didn't fully match their own.
Note that many states, including Missouri, have removed this rule from their current rules of conduct or replaced it with other conflict of interest rules. If you are looking at a current copy of the rules, check whether this section is marked as reserved or repealed, since the rule about lawyers acting as intermediaries may no longer be in effect in its original form.
