Or instead of drawing the line arbitrarily, you could say to people "you can enter only one" and let them choose. In that way you just need two concurrent contests that have clearly different skill levels involved.
2skillsets is a good start.
1 mapper makes a layout, and when he does that he will be excluded for phase 2 (other than fixing entity work in case it broke). The same for the detailer. Although he can detail his own map, it cannot take part into the contest.
You could even make an exception for that so you must detail the map of someone else. Then you actualy test 2 skills:
Creating a design someone else can continue on
Detailing a design someone else created and had barely any influence on (only from hinting for design changes)
However, this idea really sucks because why would you want to divide an already small pool of entrants?? We need to keep the contests as acessable as possible otherwise we'll have very few people actually enter.
Phase 1 had much more maps than phase 2. And as i mentioned earlier, people are less likely to detail a map they dont know its going to work. And i think this mainly decided the phase 2 contenters.
For the next 2skillsets the contest itself should be spreaded over a longer period so there is a decent voting period and the results are known before phase 2 starts.
People can still detail a map earlier if they think its good. But again, its a skill that you also should have. That risk should be rewarded.
Contest to make the best Medieval MvM map.
mvm_example can be a good contender for its shape and simplicity. Which would make the contest unfair because you cant just remake that map.