You should be fine to throw a couple
Evil Twin 
in for now. It's going to be very hard finding U/B threats right now without Dimir actually available yet.
Snappy is very expensive right now. He isn't TOO critical in your deck, and I'd consider running
Vampire Nighthawk 
. He's good tempo in your kind of deck.
The
Mikaeus, the Unhallowed 
and
GrimGrin, Corpse Born 
combo is expensive (mana wise), and in turn inefficient. You're talking 5-mana, then 6-mana, then you also need a host of other creatures in colors that don't support that kind of game plan well. As I said, you're better off with the Esper build to have access to more robust token makers -- you can merge BW tokens with control elements blue offers. If you want to talk about that archetype,
Sorin, Lord of Innistrad 
is your planeswalker.
Speaking of planeswalkers, on to
Jace, Architect of Thought 
's -2. What that does is effectively a mini
Fact or Fiction 
. That card is spectacular when it comes to filtering your deck, and putting your opponent in a position of making a choice as to what his fate will be. He does not select the cards you put in your hand, simply divides the cards into two piles. You choose which pile to take, which could be anything from 1, 2, or all 3 cards depending on how your opponent chooses to divide them. It's a great way to get through cards, help improve your chances of drawing bombs, is overall card advantage. It's almost irrelevant that your opponent knows what those cards are.
Jace's +1 is also advantageous. It nerfs your opponent's next attack, by even a couple points. In a game where you're trying to mess with your opponent's mental math and feelings of control, that's great to tack on. Milling on the other hand, you'll often hear is not effective. It doesn't affect the board state, nor does it net you any advantage. It's a dangerous path if you start trying to use milling as a primary win condition. Like I said,
Nephalia Drownyard 
is good because it takes a land slot, not a spell slot, and can be used on your opponent's turn, effectively not wasting your time to mill. It seems slow, but can be effective when used often enough as an alternate win condition.
M13 Jace needs to be used on your turn, takes a spell slot that could be something punchier, and costs 5 mana to boot. Think how long he'll actually live -- you have to use his mill ability at least 4 times say to mill your opponent out. Maybe he'll do it once before he's removed and now you've netted no advantage for yourself, but wasted a turn casting him, and now you're out of mana for a counterspell or removal spell when you really would've liked one.
Sideboarding: I'll be honest, I'm not very good at sideboarding. I never understood it, I never felt I've made smart decisions, but here's my two cents. The mirror match will ultimately come down to who is outdrawing the other player, and who is answering threats a beat sooner. This is where having options to draw your deck and find answers faster comes in handy. There will be a lot of counterspells, and a lot of big bombs. You'll find mass removal spells less effective than cheaper spot removal, and cards like
Duress 
will be good.
I'll try to help you out as much as I can! But you should think about a couple questions before going forward:
1.) Are you attached to particular cards (GrimGrin, etc.)? If so, consider adding white, and going an aggressive route. It's not bad, it'll just be different from what your original idea was.
2.) Do you like running a lot of creature threats? If so, U/B is probably not your color combination. They are they weakest in creature superiority. G/W is the strongest creature combination, with Red sitting in the middle.
3.) Do you like complex combos (Mikaeus + GrimGrin, Lantern + Lich)? If so, you should again take a look at that
Chromatic Lantern 
deck I linked earlier, or the Human reanimator deck that's cropped up. Both promote strong combo synergy. But be careful for the deck becoming too much of a glass cannon.
If you don't feel like any of these are particularly important qualities in deck building to you, I think you can go forward with the U/B deck at the moment. But like I said, it's a tough road without Dimir. When Gatecrash comes out, it will be much, much easier to try new stuff.
Cheers!
