Ok, here we go.
To start here are a few basic points which you probably know, but I feel really need to be considered.
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If you hit the point character as they call an assist but their assist has not yet touched the floor, then when the assist reaches the ground he will simply pose instead and jump back out. The assist also loses any invincibility frames that he/she may of otherwise had.
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If you do an OTG to double snap (For example off psy assist you go OTG short, slide xx snap) then you make it very difficult for yourself to get the assist kill because you cannot do another OTG lk to catch their assist (actually you can, but it can’t be OTG and the timing is strict) due to combo limitations.
Ok, as has been mentioned before; doing double snaps is a hit confirm. You need to be looking out for the opportunity, and it gets easier over time because you will know what you are looking out for. Here are a few pointers.
As the match goes on, make a profile in your mind as to when your opponent is going to call their assist. Some players are a lot more trigger happy than others, some players are smarter than others. A good tactic can be just to superjump towards them, but just block and see what they do.
In general, you won’t usually be able to get double snaps after the assist has done their move. Basically, nobody is stupid enough to leave their assist and themselves unprotected (actually scratch that, it works against scrubs who call their assist and then think doing a proton cannon is a good way to cover them… dash in short short psy snap profit!). On top of this, it is generally harder to hit confirm. Clearly not impossible, but a lot more difficult since it is more circumstantial.
A few good ways to set it up… The obvious answer being Tri jump Hk, short short psy snap. When to do it? straight after a 5 fierce combo for an easy reset. The tri-jump does NOT hit the assist, just the point character. but if they were mashing on their assist then the assist will jump in next to the player and pose. You start hit confirming from the short short psylocke, That should be plenty of time to see whether or not they called their assist.
Generally also, people are more willing to mash their assist when a) you have just done a damaging combo and you are going to go for a reset, they are more likely to panic and b) when they are stuck in the corner.
One last thing, not to do with snaps but more to do with your playstyle (but will indirectly link to snaps). I find it interesting that a lot of the questions you are asking are the same ones I was when I was just getting to that intemediate magneto stage. So I feel like I may be able to help you out 
Firstly, and sorry if you already do, but try and integrate the 5 fierce combo into your game. It’s just simply more damaging than going for the rom, and your less likely to screw up. easy way is just to a c.Hp straight off the psylocke assist (don’t let them touch the floor), and I would recommend going for this at every opportunity you can. Also, you get a free reset since they cannot roll afterwards. The only problem is you can only do this when you are right next to them, so if you are too far away then you can just lk, slide xx sjc ROM.
Secondly, learn a few simple resets. A good one is when you are doing the two lk’s down from the ROM, dash underneath them, tri jump Hk them from the otherside into another combo. Another one which is very effective is when they are on the ground, normal jump upwards, then dash forwards and call psylocke assist at the same time. The assist comes out behind them and they will be blocking the wrong way.
And FINALLY… and a lot of magneto players are guilty of this… magneto’s main poke is NOT psylocke assist. Don’t get me wrong, psylocke can be good for punishing close range mistakes. But your main source of pressure is and should be magneto.
