I feel this is the easiest set up for a Shadow Priest + Rogue, even if you don't play it perfectly. I've gotten pretty unlucky, and messed up a few times, and still won against this set up in high rated matches.

The warlock will be mounted with a fel hunter out. You can see what his buffs are since 2.3, and so you'll know if his partner is a Rogue or a Druid. The druid will be stealthed, and unfindable (unless he's terrible). Your priest should be mounted, and you should probably have fear ward.

You want to open on the lock before getting knocked out of stealth - without using sprint. This shouldn't be too hard with MoD. On Nagrand, you can simply go far around the side so you aren't seen, as many will rush straight in to find you. On Blades Edge you can go along the bottom up the back to get them.

If somehow you find the Druid in stealth, just kill him there. Blind the Warlock, and have your Priest then Silence the Warlock. More likely though, you'll 1v1 the Warlock until the Druid pops out to heal. Your Priest should hop off his/her mount to dispel you and the lock. If the Druid likes to stay in stealth until the lock gets low, you can pop BF from the start, and you'll get the Druid out a lot quicker.

Once the Druid pops out to heal, immediately blind the Warlock and sprint for the Druid. You should now blow AR (and BF if you didn't already). You should probably get a 1-2 point Slice and Dice and then a 5 point KS. Now here's a really important part: you have to pay attention to the lock and CoS before you get feared, especially during the KS. The Druid will most likely die during the KS. If he doesn't, just make sure you have plenty of energy to Shiv him. As a Prep Rogue, killing the druid should be extremely easy, hence this being the easiest match up. Soon as the druid is dead, your priest should pop out to heal and dispel, as it's very possible for a warlock to 1v2 two people who don't know how to play.