Hi, Taro,
I think if you typically only using one group the easiest and cheapest option for you is a simple pump kill switch like others have mentioned. That way you can experiment with timing and the effectiveness of preinfusion. If you do indeed think it's helpful, then you can later install a Shure Shot. If not, then you haven't wasted the money and effort of installing it.

If you are in a cafe environment and/or regularly using both groups at the same time then as far as I know your only option would be the Shure shot with separate solenoid and metering valve....which would not be cheap.

Thank you Headala,

That's kind of you.
Your suggestion is very smart.
I haven't expected that there are some way for pre-infusion.