It's great that you trust each other and have found a way to make it easier to deal with the banks. But I would still plan for the unexpected.
I have seen agreements based on trust alone become warped, painful, and difficult to sort out correctly and fairly. The presence of trust does not eliminate the need for documentation. In fact, it should make it easier since you have common ground and goals. Make it as simple or thorough as you want, but something should be in writing.
Ask yourself what kinds of circumstances might arise where you want clarity on how to respond. For example, what if one of you experiences an emergency or worse and access to the cash is needed? Do you have other family members that will need to get involved? Etc.
