Payment for the fence depends upon whether it’s a dividing fence between your two (2) Units or a boundary fence between those two (2) Units.
Check your Strata Title Plan; dividing fences are usually shown as broken or dotted lines, and a boundary fence is shown as a heavy (thick) line.
Payment for dividing fences is shared between Owners, usually in the ratio 50:50, but if you want to agree between you to a different shared-cost arrangement such as the one you suggest, then that’s fine.
Boundary fences are Common Property, so payment for replacements is made from your Plan’s Sinking Fund.