I have only written up a character.. never actually playtested but it should be fairly comparable to a normal character. You do not have to spend a feat in order to dabble in another class. Your powers are mandatorially split between both classes and you count as both classes.
you gain access to all of the weapons that both classes offer but you only
get the worst of the armor. Although you may end up with a slightly watered down version of powers depending on what class you take.