The best metal blades come from CTD machines in California. Call them, I don't have the number, I am at home today. Speak to Larry, this guy, without a doubt, knows the best blades. I purchased a set of metal cutting blades over a year ago from him. We have yet to have them sharpened and they continue to give absolutely perfect cuts.
We do use a coolant spray, so that does increase the life of the blades and improves the cut. The problem with using a coolant spray with a machine that also cuts wood is that the sawdust constantly clogs the spray heads, you have to clean them all the time. We cut wood and metal all day long, we have a separate machine for each, saves a lot of time from cleaning spray heads and changing blades.
As far as having a double bladed cut off saw and the blades you use, there is not a left-right issue. Just purchase your blades at the same time and always have them sharpened in pairs. Do not get sets of blades mixed up, keep em together.
You can not adjust your cutting depth properly if one blade has larger or longer teeth than the other. Your blades will have a logo or printing on one side. One blade will have the logo showing on the machine, the other blade will have the logo on the inside, not showing. That is the only difference between right and left blades.
I want to ad, I have been purchasing blades for over thirty years, I have talked to a LOT of "experts" about what is the best blade. Every purveyor of blades, all claim that their blade is the absolute best, no two ways about it. I purchased one pair of blades for over five hundred dollars from one of these "experts", the blades were junk, I could not use them at all, AND, they would not take them back. They now sit on a shelf collecting rust.
The ONLY straight shooter I have ever dealt with when it comes to blades is Larry at CTD. CTD has spent years finding the best blades, and that is all they sell, the best. Pricey, but not as pricey as some lesser blades I have bought over the years. Call him, you will not regret it.
Also, be sure and explain to Larry what you are doing, cutting wood AND metal. He will probably be able to sell you a blade that will handle both.
John