First off I have to say: Khundekling, this is an awesome question, and totally got my inner-geek super excited.
While I haven't done what you're talking about doing personally, I do have a few thoughts on what might happen, based on my own craft-related observations, and my background in computer science, discreet mathematics, with just a hint of quantum physics.
The possible outcomes seem to be the following: the universe will either "choose one or the other", "one layered on top of the other" (like a blend), or "expressing both simultaneously" (like quantum entanglement - which is all kinds of messy).
If I had to guess though, I'd say that it would likely be either layered on top of one another, or both expressed simultaneously because energetic quantum reality doesn't seem to be limited to functioning in an "either/or" capacity.
The reason I believe this to be true, is because multiple spells and hexes seem to be able to affect the same quality or attribute simultaneously without breaking one-another, indicating that qualities/attributes can potentially have one-to-many relationships with spellwork. It's also possible that every quality/attribute in question just has so many subtle energy variants/states that it isn't necessary to "overwrite" as a result of the overlap.
With those two possibilities in mind - what I'd be most concerned about is how the overlap is handled.
Because if my logical assertions hold any water: any areas where overlap occurs might either:
get nullified (in the case where a positive and negative are perfectly averaged out)
behave erratically (if both can be expressed depending on other variables)
have a reduction in efficacy (based on the difference between values)
get added to the mix in the wrong context (if there was no opposing value, it would just get tossed in there unopposed)
or get amplified (if they're similar typed values, and the results are additive or multiplicative)
So if you end up doing both, it's quite possible that various aspects of each spell will get either nullified, compromised, act erratically, get added in the wrong context, or amplified: and I don't believe that's going to give you the result you're looking for.
Also, if you are truly of two minds about what to do, that might hamper the effectiveness of the spells you cast too, because you're not really committed to either one.
However there's also the possibility that your subconscious/intuition may know what the best choice is already even if you don't consciously realize it yet, so you might find that if you go to do both spells: one will feel "stronger" and the other one may feel "weaker".
If that's the case though, I'd probably try starting with divination to determine what your best course of action might be, and then going into that one course, fully committed, knowing that if it doesn't work out, you can always undo it later, and then cast the opposite spell at that point (if that's an option of course).
I do have to say though, this a very juicy question and it's got me super intrigued: I obviously wouldn't want you to do anything that might have bad consequences for you, but if you decide to try this out for curiosity's sake, I'd be really interested in hearing about your results from casting diametrically opposed spells like this.
Curiosity might kill some cats, but the cats who survive usually learn some exceptionally amazing tricks along the way 😊