spell leveling up

hi everyone, i just got poe and im loving it so far. i just have a few questions on how spells level up.
1.) will a spell level up faster if you use it?
2.) how come i get a spell and it levels up individually but all of a sudden it will share exp with another spell?
thank you :)

ad 1) The only condition to level up skill gems is to have them equipped in slots.Note that 2nd weapon set also counts for that.You don't have to use these skills,though.
ad 2) Skill gems don't share exp between them.However,it is possible that several gems go up a level at the same time.Also,if you take early skill gems whose require character level 1-4 to be equipped into later stages of game they'll level up rapidly.