Hi Guys,
There's two trains of thought. Go budget and buy 1 bar and use it to death. This is the cheapest way to go, However ...
Buying just one bar means it gets used EVERY session whereas, the kite usage may be spread over two or even three kites meaning the bar wears at up to three times the rate of the kites. When you decide to sell your kite, if you sell the bar with the kite, you have a VERY worn bar which doesn't match the wear of the kite. This puts some buyers off. It also means you have a single worn bar which will be more prone to failure through wear and tear. It also means you don't have a spare should it fail. No doubt this will happen on the most perfect day of kiting and you will be off home early.

Kites are much more difficult to sell without a bar, this quite seriously reduces your potential customer base and sale price.
The other idea is to buy a bar per kite (assuming you have loads of money and want to keep everything matching and are looking for maximum resale value. Benefits are obvious. Bar per kite, little wear and tear and a spare should anything to wrong.
Alternatively, if you have a 2 or three kite quiver, buy two bars and share them between the three kites. Every other year, replace the most worn one.
If you are running a mixed quiver (not really recommended if you can afford not to), use the Duotone Clickbar as it can be fitted to every kite on the market.
DM