Shenzhen VOA if you already have a China visa

I'm a Canadian studying in HK (one semester only, so no HKID and can't get multi-entry China entry permit). If I get a single- or double-entry entry permit for a trip to Guangxi, but then want to go to Shenzhen for a separate trip first (before I return to HK and go to Guangxi), can I get the Shenzhen-only VOA or will they force me to use the entry on my China entry permit?