Choosing which overlapping spell to use?

[Ryu_Bonkosi]

I am aware of how overlapping spells work when they produce the same effect, but what I was wondering is, if you use the same spell twice, but the effect is different, do you get to choose which effect you use?

For example: The weapon enchantment "Splitting" in 'Champions of Ruin' uses the spell Arrowsplit from the same book as its base spell. If I were to cast Arrowsplit on an arrow and then fire it from a "Splitting" bow, would I use the spell's effect, the weapon enchantment's effect or would I get to choose?

[Curmudgeon]

Do you get to choose what magic applies to an arrow in mid-flight? No; your DM decides that.

The interaction of these two effects will indeed depend on the order in which their splitting takes effect.

Any missile fired from a splitting weapon, or an arrow or bolt enchanted with the splitting ability, breaks into two identical missiles before striking the intended target.
If an arrow or bolt subject to the Arrowsplit spell splits before the splitting enhancement happens, neither of the resulting Arrowsplit projectiles is the missile fired from the splitting bow, so no splitting effect occurs. However, if the splitting effect happens first, both resulting missiles share the nonsplitting properties of the original, including the Arrowsplit spell. Since both effects describe their splitting as happening "in mid-flight", it's up to your individual DM which happens first.