Approaching causal and effectual reliance through "times"
Past, Present, and Future are the three times.
Future, Past, and Present equally depend on each other.
If you try and draw a triangle with only one line or two lines, it is impossible. All three need to be in place for the totality of "time."
In this way, it might be possible to reason more clearly about how causes depend on effects.
Approaching co-emergent phenomena via phor
Your question seems to be "okay causes lead to effects, but why do causes depend on effects?"
Two people high-fiving requires a silent clap from both sides.
The beach (of sand) and the shore-line (of water) depend upon one another. To say there can be a beach without water is to have a desert. To have water without a shore-line is not a shore, but an ocean.
The nature of where they meet
Noble Nagarjuna, in Fundamental Wisdom of the Middle Way states
Forms do not meet and they do not part.
Dogen-ji made a statement of similar magnitude by saying "Space has no gaps."
I will stop there, at risk of becoming too philosophical. The teachings are meant for experiential understanding. Words will help, but like reading a menu or eating a meal, the actual experience is what counts.
For now, know that the present is dominant and that both future and past must borrow from Now in order to even come into question.