When you are given the option of an "Energy Weapon" (Power Weapon in the english version), you are choosing between one of four possible weapons - a Power Sword, a Power Maul/Mace, a Power Lance/Spear or a Power Axe. Hoping that the page references are the same this is explained on page 179 of the main rulebook (it's a hold-over from when all power weapons had the same weapon profile). Any other type of power weapon (fist, claw, hammer) is different and requires more points.
The Terminator Weapons List is split into two parts - one for replacing the storm bolter, and one for replacing the power weapon (as they're worth different points). So this means you can have a stormbolter and hammer, or sword and hammer, or even two hammers.
However - when a model has more than one melee weapon, you choose which to use each time you attack rather than being able to combine the special rules. So having two hammers doesn't double his strength again (and strength cannot go higher than 10 in any case). The benefits of having two weapons is that you can choose which to use in the right circumstances, so if you're fighting enemy with 3+ saves you use an energy claw, whilst if they have a 2+ you switch to the hammer (for example). You also get one bonus attack (still using the profile of the weapon you chose) for having two melee weapons, as long as you abide by the Specialist rule (not sure on what that would translate to).
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