Question about Dual-Class

So I have looked around for info on this but most info seems to assume you know about it already...
So say I wanted to make a Fighter/Cleric (Berserker) If I lvled the Fighter to 12 then Dual-Classed to a Cleric dose that mean I start over as a lvl 1 Cleric and lose all my Fighter skills? Then when I lvl up the Cleric enough I get my Fighter stuff back?
Also if I was lvling up my skill with say a sword as the fighter would I lose all that when I switched over to Cleric? Would it then lock me into blunt weapons until I get my fighter skills back?
As you can see I am confused as to how exactly this works.
So say I wanted to make a Fighter/Cleric (Berserker) If I lvled the Fighter to 12 then Dual-Classed to a Cleric dose that mean I start over as a lvl 1 Cleric and lose all my Fighter skills? Then when I lvl up the Cleric enough I get my Fighter stuff back?
Also if I was lvling up my skill with say a sword as the fighter would I lose all that when I switched over to Cleric? Would it then lock me into blunt weapons until I get my fighter skills back?
As you can see I am confused as to how exactly this works.
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Comments
Also you gain nothing from dual classing at level 12 fighter over dual classing at level 9. The fighter levels to dual class from are either:
Level 7 - +0.5 attack per round
Level 9 - Max fighter hit die.
Level 13 - +0.5 attack per round.
Oh and also after dual classing to a cleric, you are permanently locked to cleric weapons and will never be able to equip swords again, so you need to put your proficiency points into blunt weapons from the start of your fighter levels.
If you had placed pips in a blunt weapon as a fighter then yes you would regain all those once you gain back your fighter levels.
Personally I would also wait until fighter level 13 to dual in your case as fighters gain additional attacks per round at level 13.
Edit. And also don't put cleric proficiency points in a weapon/weapon style you have already invested fighter points in until you get those fighter points back at cleric 13. Otherwise you will lose those points.
Edit2. Too slow
In BG1 dual at level 7 fighter or 6-7 rogue.
Dualing at 12 is not a good idea over 9 or 13.
When dual classing you should aim at progressing in your second class. Trying to pick up skills from both classes will take away attention and fun out of the game.
Because you can only start with a kit I recommend using EEKeeper to transfer your kit to the second class. An easy way to do this is to start as the kit. Save the game. Go to EEKeeper. Switch the character to the first class. Save. Load the modified game.
Cleric 5 -> Mage 9 can do max level mage in BG
Cleric 7 -> Mage 8 is the highest level Cleric dual finishable in BG
However, fighter 13 requires 1.25m XP. Thief 14 only takes 880k XP and is doable fairly quickly. Mage 14 requires 1.5m XP and cleric 14 requires 1.35m XP, so those two are much slower to reactivate.
Fighter 9 only takes 250k XP and any class can get to level 10 in no time at all in BG2. Thief 10 only takes 160k; mage 10 requires 250k, cleric 10 requires 450k. So level 9 is often the most practical time to dual as you have max fighter hit dice (9d10 + CON) and reactivation is very quick.