[Edit] Pardon my getting sidetracked. In regards to your original question...iron out the wording so it works, and it should be fine. It would really come down to if the cards are costed appropriately, but it comes with a built-in downside. Sure, your beneficial spells get to hit two things, or you can steal an opponent's buffs, but it will also let your opponent do things like 2-for-1 a kill spell. Seems reasonable to me.
[/Edit]Okay, I hate to be the one to bring this up, but...you've got some bad grammar going on.
The proper conjugation of bound, in the context of being tied together (physically or metaphorically), is bound. "Bounded" refers to leaping, like
"The rabbit bounded around the meadow." vs.
"The couple was bound in holy matrimony. Later that night, the husband bound his wife with rope and they [REDACTED]."Likewise, the past participle of cast is cast. "Casted" generally isn't considered a word.
The last line is also grammatically incorrect, but that's less important because rules-wise, the wording doesn't work anyway. What it
should say to work as intended is,
"Choose any creature. It becomes bound to this creature. Whenever a spell targets one of these creatures, it targets both instead."
On the enchantment, the secondary effect should read,
"Whenever the creature bound to enchanted creature takes damage, it is redirected to enchanted creature."
I'm using "creature bound to" instead of "linked creature" because you mechanic doesn't define what's happening as "linking" - for that you'd need to change the wording from "bound to" to "linked to" - but that may be overly pedantic.
Further, you need to have your reminder text in
italics (slanted text). This is actually mechanically important. Normal text indicates rules and mechanics, while
italic text indicates flavour text and clarification - i.e. anything that isn't rules text. Reminder text isn't really a part of an ability, just clarification on how the ability works. Which is why commonly used keywords don't have it. It's used when people might not remember what a mechanic does, and removed when a keyword has been used frequently enough to be considered common knowledge.
Lastly, Jéské Couriano is correct. You need to either include when/how the ability activates, either with a cost to activate (in which case the reminder text will include
"Pay [whatever the cost is]." at the start), or a specific situation under which it activates (in which case the reminder text will include that situation at the start, such as
"When this creature comes into play, choose a creature." or
"Whenever [thing happens/phase of turn/etc],"). If it's an event other than coming into play, you'll probably also want to include a condition like,
"[...] if this creature is not already bound." or you might want to make a "may" condition and give players the option of when to use the ability. Also, if you go with "comes into play" for a condition, then your enchantment will need to specify that a creature is chosen then and there. Something like this...
Just a quick mock-up, obviously. You actually wouldn't put the "bound" or its reminder text in quotation marks either, but I was just doing this quick and dirty, and didn't even save the card to edit and fix. You can see how it
should look in cards like
Aspect of Gorgon, where the ability is just a part of the text line, and then clarified below. That's a part of being a keyword.
