It would be very simple:
• One finger: Point.
• Two fingers: Click 1st button.
• Three fingers: Click 2nd button.
On more modern devices, there’s support for detecting when the finger is hovering above the display.
But I don’t have that on my Galaxy S3 mini.
And still, this would allow more than one button, e.g. to map the second one to the eraser in drawing programs.
Even cooler would be simulating a “wheel” controlled by the distance between the first and the second finger. That would make it very easy to control the brush size on top of the opacity at the same time. (Can Wacoms even do that?)