any spell/food/potion that boost your skills cancels the effect of any other previously used food/potion/spell to boost your skills, so if you use for example a berserk potion to get an extra +5 of axe/sword/club fighting and then use the blood rage spell you are going to loose those extra +5 you got from the berserk potion, doesn't matter if the potion had an effect of 10 minutes on your character and you used the blood rage 20 seconds after you used the potion.
Regarding the shielding skills, I'm not completely sure but I've read somewhere that if you are killing more than 3 monsters at the same time your shielding skill doesn't have any effect at all, that's why many knights prefer to use a two-handed weapon, and only use a shield when you need that extra % of elemental protection using an imbuement on your shield