Less worst scenario in a Nexus ?

I have limited scaled experience, but from where I sit, I'd say:

Should they have more time for Product Backlog Refinement (how much ; where to stop ?) in order to reduce the dependencies

I would say so. However the question I would be more interested in asking is: what dependencies are those that you cannot remove/fix after one or a couple of sprints worth of time? And are they feature teams or component teams?

or is it better to merge the 2 Scrum Teams, creating a huge Dev Team of about 16 to 18 people ?

I never recommend teams made of more than 10-12 people, and they're already too large! 

Would it be feasible to mix the two Teams for cross-functionality and still keep two teams? As Eugene mentioned, Feature Teams.

What type of dependencies are they?

If you have skills dependencies then you may not have enough cross-functionality across the entire Nexus. You may need to take a hit on velocity for a few Sprints to train and upskill some people from all teams.

I wouldnt merge the team into a giant team, you could just end up with dependencies within the team and a relative decrease in velocity. But depends on the nature of the work. Smaller numbers of people work together more efficiently. It might be worth approaching from the opposite direction and asking, "if I split these 2 teams into 4 small teams, will it give me more flexibility to slice stories and eliminate dependencies". The smaller teams should also move faster once they settle.

Very difficult to say without details of the nature of the dependencies.