I agree with Jingles from the standpoint of "just get the lights to do it", which is ultimately what your end results are.
The whole purpose of the 3D Visualizer is to help you design, so I get that point of view too.
NOW, let's say you have two fixtures that you have addressed identically. You may have able to tell one of those ELation Design Spot 250's to "invert pan", as typically that's what people want to invert is pan, not tilt(but who am I to question you). There's nothing wrong with this, BUT in the 3D Visualizer, you have to add a second fixture with the same addresses and I suppose there may be an option to "invert" either X or Y. I can't speak authoritively on that because I haven't done this yet(nor plan to, I have enough addresses to spread stuff around).
Note, there is absolutely NOTHING wrong with having fixtures match up addresses in DMX, which is a nice thing and can be a huge time saver in design, so don't let me tell you you NEED to change things. But hear this out:
In MyDMX(the main part), there is no way of telling MyDMX that you have more than 1 fixture at a particular set of addresses. Likewise, I don't think any other DMX package or hardware knows this sort of thing either, that you have two fixtures sharing addresses(because we don't have sub-addressing either).
So, let me dig deeper so you can get into my head and my thoughts:
Why? Seriously, why bother sharing addresses unless they are wash lights? Chances are, unless you've got loads of fixtures or tons of lights already using mega amounts of channels, then why share?
So, right now you want to invert pan. OK. Fine. But, if you give your lights unique addresses, you have total control over both. Do this scene invertered(say, they point OUT) then another scene where they CROSS(OK, inversions are OK), but you have another scene where you go from down to up or up to down, and your inversion would be goofy this time. Or, you can have the lights move from left to right or right to left(and this would not be inverting).
When making decisions such as pan inversion or sharing/duplicating addresses. I've seen many many times here on this forum where some guy has say 4 lights in a shared address configuration, then gets all snotty when he's done tons of programming and then wants to "break" it and give each fixture its own address but doesn't want to repeat his programmng work. Sorry, MyDMX doesn't accomodate that. I also get that people don't want to replicate their work, but sometimes it can't be avoided. I make things for myself where I end up having to duplicate some or all of the work, but I don't whine about it(at least not here). My whine is typically "crap, this means I'm gonna have to do this, that and the other thing over again.... well, whining ain't gonna get things done, dive in!"
As far as the 3D Visualizer, if you tell the MyDMX program what profile you're using(which helps us mere humans relate to fixtures better), the 3D Visualizer knows ENOUGH to start doing the animation. No profiles means this won't work so well.
So, in short, if you just want them to mirror(pan invert) each other, just do it at the fixture.
If you want to do it in MyDMX, you've got a whole new set of things to take into consideration.
Please describe how you set up MyDMX, that may help us know where you are in regards to this project.