I have a double entry visa valid 6 months, can i still used a transit G to spare 1 entry ?

Hello,

I already have a unused 2-entry visa valid for 6 months.
I will use the 1st entry on Sept. 24th in Shanghai. I will leave China on Oct 1st. I was wondering if i could go to Seoul, in South Korea on 28/29 and 30th of September and then back in Shanghai on Sept 30th, to leave back to europe on Oct 1st. Do i need to use my second entry of my entry permit on Sept 30th or could I use a visa transit for that quick stay ?

thanks for your answer,

best,
Fanck