From this question,we know that Kartikeya has two wives,Devasena and Valli.
However,this question is mainly on Devaseena as well as another goddess, Shasti. She mentioned in the Devi Bhagvata Purana as so:
4-22. Nârâyana said :– O Child! The sixth part of Prakriti is named as Sasthî. The Devî Sasthî is the Presiding Deity of infants and children; She is the Mâyâ of Visnu and She bestows sons to all. She is one of the sixteen Mâtrikâs. She is known by the name of Devasenâ. She performs Vratas (vows); She is the chaste and dearest wife of Skanda. She decides on the longevity of children and is always engaged in their preservation. So much so, that this Siddha Yoginî always keeps the children on her side.
Also, chapter 228 of the Vana Parva of the Mahabharata just says this:
The Performer of a thousand sacrifices then thought of Devasena, whom he has rescued before. And considering that this being (Skanda) was undoubtedly destined to be the husband of this lady by Brahma himself, he had her brought there, dressed her with the best apparel. And the vanquisher of Vala then said to Skanda, 'O foremost of gods, this lady was, even before thy birth, destined to be thy bride by that Self-existent Being [Brahma]. Therefore do thou duly accept her lotus-like beautiful right hand with invocation of the (marital) hymns.' Thus told, he duly married her. And Vrihaspati learned in hymns performed the necessary prayers and oblations. She who is called Shashthi, Lakshmi, Asa, Sukhaprada, Sinivali, Kuhu, Saivritti, and Aparajita, is known among men as Devasena, the wife of Skanda. When Skanda became united to Devasena in indissoluble bonds of matrimony, then the gods of prosperity in her own personal embodiment began to serve him with diligence.
Although these all point to Devasena being Shasti, I have a few points that contradict these texts.
In chapter 223 of Vana Parva, Devasena told Indra that she wanted husband with the following qualities:
O hero with long arms, I am Avala (weak) but my husband must be powerful. And by the potency of my father's boon, he will be respected by gods and Asuras alike. That manly and famous and powerful being devoted to Brahma, who is able to conquer all the celestials, Asuras, Yakshas, Kinnaras, Uragas, Rakshasas, and the evil-minded Daityas and to subdue all the worlds with thee, shall be my husband.
However, the Devi Bhagavata Purana contradicts this:
23-24. O Nârada! In ancient times, when the Daityas dispossessed the Devas of their positions, this Lady was elected as a general and got victory for the Devas; hence She was named Devasenâ. Hearing the words of the King Priyavrata, Devasenâ, who fought for the Devas and was all good to the whole world, said :–
Here,as per the Devi Bhagvata Purana, Shasti stronger (she led the Devas like Skanda) as compared to the weak Devasena of the Mahabharata.Also, Shasti got the name Devasena after a particular incident, unlike the Devasena of the Mahabharata who is named so after her birth.
Also,who is Shasti/Devasena's real parents and how does the concept of Shasti being Devasena tie with Valli? As per Tamil folklore,Devasena is said to be the reincarnation of Amritavalli,who,along with Sundaravalli (the previous birth of Valli),was born of Trivikarma (Vishnu)'s tears when he saw Nataraja's dance. When Amritavalli reincarnated as Devasena, she was adopted by Indra.
As per the Mahabharata, Devasena,as well as her sister Daityasena, was born to Brahma.In the Devi Bhagvata Purana too, Shasti is described as being the mind-born daughter of Brahma.
In addition to that, if Devasena is Shasti, why isn't she worshipped under the name of Devasena?