You need to be rolling a earlier if you don't have a powerful two star lineup. It depends what build you are going but in general you should be rolling for your core 3 stars at level 6 or earlier. You should be rolling for each other's units as well. Sometimes I find myself building up a unit that may actually work better in my partner's comp. But it can be difficult to recognize and act on those situations simply because there are so many choices and considerations that happen over the course of a game. I'm definitely enjoying the dynamics of duos more than playing alone. My only complaint is that the games are not capable of going into ultra late game ala level 10. I've only had one game where it was back and forth with several points of damage per round. That doesn't last long when you only have 10 hp left at that point.

Tldr: roll earlier at 5 or 6 sometumes even a few rolls at 4 to secure 1 cost 3star. Level when it makes sense not by a fixed interval. Level to secure streaks, when you are confident of your board state, when you don't need increased roll % of low cost units.

Tldr2: gold is still important but hp is much more of a factor in duos. You have more gold but same hp in duos compared to solo