My friend pointed this out to me which Barks wrote it on their website :
同シングルには全4曲を収録。カップリングはAKIHIDEとSHINPEIそれぞれが作詞作曲を担当した ほか、DAIGOとのツインボーカルを展開するなど、メンバー各々の個性が色濃く反映された仕 上がりだ。
https://www.barks.jp/news/?id=1000194148
Both B-sides of I love my daughter ie. Fuyu no Hana & Tsuioku will include twin vocals which DAIGO will sing alongside with each member who wrote & composed each B-side respectively
In this case :
Fuyu no Hana - alongside with AKIHIDE
Tsuioku - alongside with SHINPEI
Barks wrote that by doing this , it strongly reflects the individuality of each member
For now I don't know what does twin vocals mean , it could mean it's a duet or only AKIHIDE and/or SHINPEI be featured vocally in the chorus of each B-side alongside with DAIGO
Only time will tell !