24h twov doesn't work in Shenzhen if you enter/leave from/to Hong Kong/ Macau.
You either get a 10 years visa or a 5 days VOA if you wanna visit Shenzhen.
Shenzhen VOA seems to come and go for certain citizens (e.g. US and UK). The thing is the cost is pretty much the same as better visas, so I could only advise using it in extremely important last minute situations (e.g. Tencent wants to award your company with a 10 figure contract, and requests an in person meeting).