Veronica Lodge, Jason Grace, and Scott Summers are in on this:

You may not realize it since we're an eclectic and boisterous bunch and surely that can only be indicative of a single origin, but we come from many places. I've learned that my worldview and my colleagues' worldviews are radically different. We may be working toward the same goal, but we still need to interact and understand one another.

My colleague Jason comes from a nation that was once opposed to another nation until a greater threat made them unite. Knowing that they would need strong leaders to weather the circumstances, they arranged an exchange of leaders to learn about each other's culture and establish a bond of trust. It was successful, which I believe is encouraging.

Where both of your nations complement each other best is in the tradition of strong matriarchies. The Sea Clan is more progressive, but the Mountain Clan can teach them about upholding tradition. Valuing where you've come from is just as important as where you're going.

As for for how to implement cultural exchange:

Maintain both capitals, rotate between them for visibility.

Nobles travel with these envoys to mingle.

Patronize artists from both nations to celebrate each other's cultures.

Have festivals to show that you're unafraid of what might come of them.

Maintain the same celebrations and holidays.

Set up Sea Clan-influenced schools in the Mountain Clan and vice versa. Set up merit programs for each, with promised accolades upon completion.