In such case, the possibilities:
1. If your application is approved the same day, your wife may apply for FLR(M) in person the same day.. To do so, you will need a separate appointment for her FLR(M) application (with some time difference between your and her appointment). Or she may apply at any other day at a PEO.
2. If your application is approved the same day, she may choose to apply for FLR(M) by post any day before her current leave expires.
3. If your application is not approved on the same day and you do not get a result before her current leave expires, she must make a FLR(M) application by post before her current leave expires.
As for the SET(O) form,
read section 3 carefully.