In full-blown collaborations and partnerships, decision making is done together or jointly. The underlying belief is that two heads are better than one and that a strong decision with buy-in on both sides is one that is based on agreement. It has been found that the process used to arrive at the decision has an even greater impact on the partnership relationship than the decision itself.
Joint decision making implies and requires that both parties see and treat each other as equals. They have a tacit agreement to share the power and control. This sharing of power can be challenging when the parties are not accustomed to making joint decisions and/or when the parties do not have the same amount of influence, or resources outside of the partnership.
One of the reasons why people shy away from trying to reach a consensus decision is a lack of knowledge about clear methods or tools for checking consensus. Although attempting that aim, they may be using models that make achieving consensus difficult.
Generally a good rule is that any significant decision should be arrived at together by all primary partners. However, as roles and responsibilities are also going to be assigned or divided, it makes sense that some decisions do not require participation from all partners. An alternative would be to consult them, and/or report back with the decision taken.