Both High and Low needle adjusts fuel to the carburator. Why can't you keep one wide open and adjust the other to control how much fuel goes into the carburator?

Each needle controls the amount of fuel entering the engine at different RPMS, low needle = low rpm, high needle = mid to high rpm.

The carburetor has a high speed circuit (main needle) and an idle circuit (idle needle). They need to be adjusted separately because they control the fuel under two very different conditions. Both come into play at part throttle. This is similar to most other small engines such as chain saws, weed whackers, a lot of mowers and so on. It's also similar to carburetors used on cars & trucks except those usually had a fixed jet or some sort of metering rod/jet system for the high speed.

I've seen a couple model engines that were throttled but had simple carbs with no low-speed needle. They were both fairly small, .074's I think. The trade-off is a less reliable idle.

Having two screws to adjust is no big deal. Just follow the instructions and it'll go well.

nrad ..An engine running at low RPM's runs better on a leaner mixture..so when you are at an Idle the low speed needle is the one controlling most of the fuel flow(if not all). And when you are wide open it has almost no effect.- BW

http://www.smrcc.net/Tips/Engine/2-stroke_engine.html

this is from an old Model Airplane News insert. it was in a small booklet form that was removable. I often print it out for new flyers, and keep a copy in my flight box.
It shows about all variations in 2 cycle motor carb set ups.

The functions of the carburetor are:

1) To control the amount of power produced by the engine by modulating the amount of air-fuel mix that it can aspire. This is achieved through the position of the throttle’s barrel.

2) To provide the engine with a proper mixture of fuel and air for any position of the throttle.

Number two is easier said than done, because the engine needs a different mix proportion for each regime of rpm and load.
For idle, the glow engine needs 3 parts of air per 1 part fuel
For middle rpm’s, the glow engine needs 4 to 5 parts of air per 1 part fuel
For max rpm’s, the glow engine needs 6 parts of air per 1 part fuel

A perfectly modulating carburetor will be able to provide the perfect mix for each rpm regime, besides compensating for changes in air temperature, pressure and humidity (that is what the computerized fuel injection system of your car does). Unfortunately, such carburetor will also be complicated, expensive and heavy.

For our simple RC engines, which are fed by more rudimentary carburetors, the proper mix for each rpm is a fine balance that is difficult, but not impossible to achieve, if all the engine’s parts and fuel are working properly.

A well-tuned and good carburetor will provide a proper combustion through a balance between fuel and air that is good for idle, transition and max rpm’s.

The vacuum created by the flow of air through the Ventury next to the throttle is the force pulling the fuel out of each spray-needle into the mix.

Think of the low-end needle like part of a little carburetor that mixes the minimum airflow of idle with the proper fuel amount.
That ideal fuel amount is what we adjust by turning the low-end needle.

Think of the high-end needle like part of a bigger carburetor that mix the max airflow of full rpm’s with the proper fuel amount.
That ideal fuel amount is what we adjust by turning the high-end needle (and the low-end needle, which also provides the small amount of fuel that is proper for idle).

The middle rpm’s regime is satisfied with the combined mix from both carburetors, the little one for idle and the big one for max rpm’s.