a) Wouldn't a class be spreading too thin over these base & subclasses?
Possibly. It depends on the class and on the creativity of Obsidian I guess.
b) What does general/base class consists of? A bit from each subclasses? Why not drop it and make a third subclass instead? or drop it and make the 2 subclasses more distinct?
Perhaps it's worth giving some examples of kits in BG2 to give some idea of how this might work. BG2 had the Fighter class, and the generic Fighter had all the abilities of a Fighter with no bonuses or penalties. You could then pick one of three kits: Berserker, Kensai or Mage Slayer (or Hunter, I can't remember). The Berserker got an additional ability that the base Fighter didn't called, unsurprisingly, Berserk, which temporarily gave a buff to combat ability and immunity to various afflictions. In exchange for this, the Berserker was prohibited from raising their proficiency in ranged weapons past the lowest level.
The Kensai was a little more complicated. It gained a passive bonus to accuracy and damage every X levels, and a special per rest attack that did huge damage, but in exchange for this it couldn't wear any armour or use several different categories of items.
So what does this tell us about subclasses in Deadfire? Possibly nothing of course, but my guess is the following. Each class's base version will be fairly similar to their current incarnation in PoE. Each subclass will then give various bonuses and penalties (for example there's a ranger subclass that doesn't have a pet, but my guess is they'll be better with their ranged weapon as a result). On top of this, I suspect certain class abilities will be off limits to each subclass (the petless ranger won't be able to take pet improving abilities obviously) and perhaps will have some new, subclass unique ability choices to replace those they lose access to (BG2 didn't really have this because most classes didn't get many abilities at all).