Can I buy Two joker's and use them with two cubes from my hand? For example. 3 j j 6. Or is it necessary to have two cubes for each joker?
From these rules:
A joker that has been replaced must be used in the player's same turn with 2 or more tiles from his rack to make a new set.
So if you have retrieved a joker from another set, you must place it with tiles from your rack (this can also be a joker). Which could be interpreted as place the retrieved joker in a set using only tiles from your hand.
I know it does not explicitly says you can not use tiles from the table, but the "with 2 or more tiles from his rack to make a new set" is clear enough for me.
Of course, you can always add house rules. Or the rummikub set/edition you have may have different rules.