Generally speaking, two flute is for deep slotting, especially in softer materials with large depth of cut and high chip loads, where you want additional chip clearance and room for the chip to get flushed out (and not get recut). And because of the reduced cutter engagement, they wiggle around less during a plunge cut. I believe these are the main reasons to use two flute.
If all else is equal, a four flute is fed twice as fast and will cut twice as much material before dulling. Finishing cuts taken on the side of the cutter is typically done with four (or more) flutes because you can feed faster and still get a good finish. Five and six flute is not uncommon for this.
Conventional wisdom says four flute is stiffer than two flute, but depending on how they're ground two flute can be stiffer than four flute because the diameter of the "core" can be much larger. So it depends on the cutter.
Improvements in materials and flute geometry has allowed production machinists to start adding flutes in applications that were once the domain of two flute cutters. Three flute are very commonly used, decreasing machining time by 1/3 with no negative consequence. There is an application where I'm machining a soft material (plastic), and the toolpath is entirely inside the part, and the cutter is plunging where I'm actually using a specially designed five flute. This is uncommon and I didn't start there.
For your application (assuming you have a small home shop type setup and are cutting stainless):
The primary advantage of two flute to you will be that two flute will wiggle around less during the plunge, gouging the sides of your slot less. If you pre drill holes this becomes moot.
Normally I suggest using a bit smaller than your slot. This is because, while plowing, the force on the front of the cut tends to deflect the cutter to the left, gouging your part and you need clearance to clean this up. However, in your case, I don't suggest going under 1/8" if you're going 1/2" deep.
You need to use carbide for this because it is stiffer than HSS. You should pre drill your plunges on both sides of the slot with a 3/32 drill. And here is the trick that is going to make this successful: take a light depth of cut. Perhaps no more than .010" per pass. Turn it relatively slow for a 1/8" carbide to prevent chatter (due to the length), probably about 1200 RPM. Feed hard enough that it don't squeal or work harden, but not so fast it deflects to the left. Probably around 5-10 IPM. Use a squirt bottle of coolant or compressed air to keep the slot free of chips. It is going to take a while.
Oh... And I'd use a four flute, even though this is deep slotting.... (you can get away with this because of the light depth of cut)
