If all you're going for is sliced, I guess you could cut them accross the long side. I've never done that myself...it will leave you with the flat cut (thinnest section) having a small amount of the point cut (thickest section) remaining on the flat. Then, you'll have the point cut with a layer of the flat underneath it. There are two overlapping muscles in a whole packer brisket.
The flat cut is typically what folks slice after cooking to about 180* and wrapping in foil/towels to rest for several hours. It is a leaner muscle with less intermuscular fat than the point cut, and is a fibrous muscle construction, which makes it very well suited to slicing. If conditions are optimal, and a minimum of 195-200* internal temp is reached, the flat can be pulled instead of sliced, but it's not the best idea for the novice brisket smoker as it can become somewhat dry if you don't use the appropriate methods.
The point cut is what folks typically cook to 160-180*, then, cube it up and sauce or re-season and double-smoke for burnt ends. It has much more intermuscular fat and a tubular muscle construction, which gives it it's characteristic texture. Or, it can be cooked to 200-205*, wrapped in foil/towels and rested for pulled brisket.
But, if you're just going for a basic brisket smoke and slicing, cutting the packer in half won't really hurt anything as far as the finished product. The point will be juicier than the flat and have a more tender chew if both are taken to the same finished temp.
Hope that helps...didn't mean to go into so much detail, but this should give you a few options to consider if you want to do some sliced and some pulled brisket. What ever you choose, it will all be great eating...we love brisket here.
Oh, best way to go for seasoning is no dry rub, IMHO. Just salt, pepper and garlic...brisket has a great flavor, so you don't want to cover it up. Trim the edges a bit to remove the really tough, grey colored tissue, trim some fat off and score the fat cap (cross-hatch), then smoke it fat-cap up to self bast @ around 225*. Low & slow is the basic key to nice and tender brisket.
Eric
EDIT: forgot to mention this, but you need to be aware of the baffling effect that larger cuts will have when you load up a vertical smoker. I suggest puting the heaviest pieces on the lower/middle grates, and the lightest pieces on top. This should give you a bit more even cooking times. Also a grate rotation (180* turn) may help as well, depending on how evenly your smoke chamber heats from side-to-side and front-to-back.
