I'd ask Matt Tabak, but this is an original design so he can't look at it. I made a green/black Instant that reads "If a +1/+1 counter would be placed on a creature this turn, instead put a -1/-1 counter on that creature. / If a -1/-1 counter would be placed on a creature this turn, instead put a +1/+1 counter on that creature." As it stands, I think the card actually does nothing, since it has two abilities that replace each other. If they were the same line of text, would that change things?
My reading of this is that the two abilities would ultimately have no effect. Say you tried to put a +1/+1 counter on something with Battlegrowth. That event would get replaced by the first replacement effect, which would try to put -1/-1 counter on the creature, only to have the modified event get replaced by the second replacement effect, putting a +1/+1 counter on the creature instead.
If an effect would try to put a -1/-1 counter on a creature (with Scar, for example), the same thing would happen in reverse and you’d still end up putting a -1/-1 counter on the creature.
Unfortunately, there isn’t an elegant way to combine the two. The best way I can think of to do what you want is with activated abilities that set up delayed replacement effects, such as:
0: The next time a +1/+1 counter would be put on target creature this turn, put a -1/-1 counter on it instead.
0: The next time a -1/-1 counter would be put on target creature this turn, put a +1/+1 counter on it instead.
Of course, since the activations are at-will, they essentially work the same as something like this:
If an effect would put a +1/+1 or -1/-1 counter on a creature, instead choose one — Put a +1/+1 counter on that creature; or put a -1/-1 counter on that creature.
Honestly, the most elegant thing might be to only use one half of the ability rather than try to make the card do both.
