重なる: overlapping schedules and repeated efforts
Distinguish the reported overlap of meeting and business-trip schedules from the condition linking accumulated small efforts to major achievements.
Schedule overlap and accumulated effort
重なる (かさなる) means to overlap or accumulate. The meeting and business-trip example concerns overlapping schedules, while the effort example concerns small efforts being repeated.
First identify 日程, the schedules, and 努力, the efforts. Then check how each relation is stated: the overlap already happened, whereas 重なれば introduces a condition about repeated effort.
A reported overlap and a condition about results
「会議と出張の日程が重なってしまった。」 (The meeting and business trip schedules ended up overlapping.) The schedules of the meeting and business trip are what overlapped. 重なってしまった reports that result; it does not establish that either activity was completed.
「小さな努力が重なれば、大きな成果につながる。」 (Repeated small efforts lead to major achievements.) The Japanese 重なれば means if the small efforts accumulate. The sentence describes their connection to major achievements rather than reporting that someone has already attained such an achievement.
Keep the subject and the condition in view
The first example names the 日程 of two activities. The second links repeated 小さな努力 to 大きな成果. Reading both uses solely as conflicting schedules would lose the second relation.
The effort sentence supplies no required count or duration. Preserve its conditional form when explaining the relationship, and keep it separate from the past result reported for the schedules.
What overlaps or accumulates in the two 重なる examples?
Show the answer and why
The meeting and business-trip schedules actually overlapped. In the other sentence, small efforts accumulate; 重なれば presents the condition under which they lead to major achievements.