For a dual output air pump I would get two things:
1) A plastic airline Tee to convert the two outputs to one
2) A gang valve with at least two outputs
With the gang valve, one line would go to the air stone and one line would serve as a “bleeder valve”. This would allow you to dispense excess air (that is not required by the air stone) and not put too much back pressure on the pump (which can lead to a shorter pump life).
What kind of filter do you currently have on the tank? If it is an HOB (hang on back) or internal filter, sometimes lowering the water level will increase the surface agitation.