Mirror matches are always a pain in the ass. Too much is based on RNG, the other rogue finding the other, vanishes bugging etc etc. There are a couple ways to engage a mirror team. If you notice that the other rogue does not wish to start on your mage, you can have the mages duel for a while. 90% of the mages (to my knowledge and experience) being frost, it shouldn't be a problem for Arcane spec. If we manage to dispel the Ice Barrier, you should ALWAYS start off on the opposing mage with Ambush -> Mutilate -> Vanish. At this time you will have done serious amounts of damage and you can forget about targeting the other rogue. If the rogue pops out, Blind / PoM-Poly him and finish off the low HP mage who has already used his Ice Block. Then there are, once again ladies and gentlemen, options A) and B): A)You get sapped B)You get a sap on the opposing rogue
A) This is the only matchup in which trinketing the sap is the only viable option. You must trinket / vanish instantly and hope that you won't get found again. Because if you do, it's over and if you don't trinket, it's over.
B) If you land the sap, you should waste no time and start with a Garrote on the other rogue, instantly popping Evasion and preferably landing a Mutilate. Then vanish and let your mage put pressure on the rogue while he is bleeding for 18 seconds. You can stay off distance for that amount of time, be aware of FoK and other relevant things which might get you out of stealth. Just as the Garrote is fading away, re-apply it or start with a cheap shot (The rogue WILL trinket here) -> insta kidney shot and land the kill.
Trinketing a Sap? By far the worst thing to do against another Mage Rogue team. If the Rogue is hovering around you, he will more than likely re-sap you. If you DO manage to get the Vanish off, the other Mage will more than likely Pet Nova you out or spam AOE's which will definitely get you out. If they don't, as soon as you come out, you're sitting through a full Stun Lock, the Rogue cloaks so he can't be PoM sheeped and you die. Other than that your strat seems alright. Against other Rogue teams you should never trinket a Blind or a Sap unless you KNOW you can kill something in one global or before you get CC'd or killed.
You don't have a strat for if the rogue opens and vanishes repeatably on your mage with ambush mutilates. This is what any good rogue mage team does vs a mirror...
Well I guess if you know that the opposing team is going to start on your mage, I'd say hover near your mage yourself and get the opener on the rogue as soon as he pops out. I know that frostmage/rogue has a slight advantage on this matter since my mage doesn't have Ice Barrier.
Every good Mage Rogue vs a Mirror will always have the Rogue open with a Cold Blood Ambush and Mutilate (Pyro Rocket if available) to force as much damage as possible on the Mage forcing a block. If it's a Frost Mage, the Rogue will just wait for his barrier to drop by his partner spell stealing or damaging it down. Being Arcane it is even more simple for the Rogue since he doesn't have to worry about Ice Barrier. The Arcane Mage will take a lot more damage and will die easier. Pretty simple.
arcane mage will have the same problem imo especially with his ( propably ) mage armor to avoid the longs sheeps / cs from a rogue , just ambush goes likes 5k 4combos cold blood evisr8 will land 10k , trust me a mage rogue can force an arcane mage to use ib much faster.
You said "if we manage to dispel the mages ice barrier" How do u dispel with mage rogue? :S do u mean spell steal it? With my mage rogue team (i no its shit rating lol) we just try burning the other mage and spell steal his ice barrier and ill blanket cs him when rogue opens with ambush etc so he cant blink and if we get lucky we land a kill then downhill from there