Is this possible, for example; i am a dualed Berserker/Mage kan i do both the planar sphere and nalias castle ?

yes. start with Nalia's Keep, clean it and after that just DON'T report back to Nalia and proceed with whatever you want to do (not necessarily the Planar Sphere). If you want to, start with the Planar Sphere quest-line (Tolgerias, Cowled Wizards, Umar Hills->Valygar etc.) and FINISH this one (inherit the sphere from Lavok) and then hasten back to the keep and speak with Nalia...everything should be fine and you'll end up being master of two s'holds (just like me, hehe)

An important thing here is do not report back to Nalia(or even do the quest yet if you wish) until after you regain your fighter class abilities. I don't *think* you will get the keep if only your mage abilities are active.

Oh that might be, dunno. I can only guarantee for multi-classed Fighter/Mages...

Can you own a stronghold in BG2?!
Do you get taxes, servants, militia, catapults, farms and other stuff with it?

:)

A dual classed fighter mage can also get both strongholds.

Now as far as your expectations of strongholds, Lukki, well, they're a little high. This isn't an empire simutaltion builder or something of the sort. Think of strongholds as the main achievement a class can have. For example, for a fighter, it's a keep, for a thief, it's a guild, for a bard, it's a playhouse. Each class has a unique stronghold, with 2 exceptions: fighter-barbarian-monk=keep, sorcerer-mage=planar sphere. Every class gets to do every stonghold quest, but you only get a stronghold as a reward if your class matches the quest correctly. Otherwise, you get some other reward. For example, any character can do the Mar Vaer quest, but you only get his guildhouse as a reward if you are a thief(multi-dual-regular).

Now that the intro's done, what's the purpouse of a stronghold. Well basically it's a side quest to obtain them. Once you get a stronghold, you need to complete several other side quests, each giving rewards of some sort, whether experience, money, items, etc. None of the side quests are very involving.Some may give you weekly money, a weekly batch of potions, a regular income of loot, it doesn't amount to anything major as their are enough great items and gold in the game it makes the rewards minimal at best. But don't get me wrong. When you first get a stronghold, it should make you smile with accomplishment.