Ex. We have two divisions (A + B) with 5 teams in each of them.
Each year all teams in division A will have to meet one team from division B on a rotating basis.
TeamA1 - TeamB1, TeamA2 - TeamB2, TeamA3 - TeamB3, TeamA4 - TeamB4, TeamA5 - TeamB5
TeamA1 - TeamB2, TeamA2 - TeamB3, TeamA3 - TeamB4, TeamA4 - TeamB5, TeamA5 - TeamB1
That is easy enough to build.
The issue is if each team will need to meet two teams from the other division where one of the games are "fixed". With fixed I mean that they will always meet that team.
So if TeamA1 - TeamB1 is fixed then TeamA1 needs to meet TeamB2 year 1, TeamB3 year 2 and TeamB4 year 3
Let us then say that TeamA2 - TeamB2 is also fixed then TeamB2 will meet TeamA1 year1 etc.
I hope this makes sense