My wife is a Chinese national and we have an 11 year old son. We came back to the UK for our son to go to school. My wife has a double entry visa, as we havn't applied for a spouse visa yet, as we are not in a position to, due to me not working. We intend to apply for a spouse visa after I get a job that meets the threshold. My wife has been here for five months and will return to China for three weeks. We want her to stay for 6 months on her return, then go back and apply for a spouse visa. Is it possible for her to use the double entry visa within the same year?
This answer expands on points already made briefly in comments on your question. There is no inherent problem with using the visa twice in the same year, but there are a couple of problems with the proposed travel pattern.
The first problem is that your wife may appear to be living in the UK and visiting China rather than the other way round. According to Standard Visitor visa - Eligibility:
Your visa may be cancelled and you may get a long-term ban on visiting if your travel history shows you’re repeatedly living in the UK for extended periods.
The other potential problem is that a visitor is supposed to have ties to their home country that ensure they will return home. In order to spend so much time in the UK your wife may have abandoned social and economic ties in China. Obviously in the long term your wife wants to live with you in the UK, but the more she has an established life in China, with somewhere to live, job, etc. the less likely she appears to be to overstay at the end of a visit.
The risk in your current plan is that she could be banned from visiting, and not able to spend any time in the UK until you can get the spouse visa.
I recommend studying the search result linked in a comment by phoog: [uk] visits frequent successive. Although there are many useful answers there, pages 17 and 18 of Visit guidance are very relevant, especially the section headed "Frequent or successive visits:how to assess if an applicant is making the UK their main home or place of work". The document tells UK Home Office staff what they should be considering when deciding whether to admit a visitor.