First of all if you read the Qur'an starting with al-Fatihah moving to al-Baqarah and so on this constellation isn't the first were the verbs "khalaqa" خَلَقَ and "ja´ala" جَعَلَ are used for things that have been created by Allah. Therefore tafsir scholars here often refer to earlier explanations.
In tafssir al-Qurtobi -here in Arabic- on this verse you may read:
In the following I will translate from Arabic language, if I haven't added a source of the translation, the translation is of my own take it carefully
وجعل هنا بمعنى خلق لا يجوز غيره قاله ابن عطية
And "ja'ala" here means "khalaqa" no other synonym can be used this was said by ibn 'Atyah (see the original statement here in Arabic)
Al-Qurtobi added (same reference):
قال النحاس : جعل بمعنى خلق وإذا كانت بمعنى خلق لم تتعد إلا إلى مفعول واحد
An-Nahhass said: "ja'ala" is -here- in the meaning of "khalaqa" and if this is the case it doesn't require more than one (grammatical) object.
In the English translation of tafsir al-Qurtobi (VolI., p 180)of verse (2:22):
الذي جعل لكم الأرض فراشا والسماء بناء وأنزل من السماء ماء فأخرج به من الثمرات رزقا لكم ...
[He] who made for you the earth a bed [spread out] and the sky a ceiling and sent down from the sky, rain and brought forth thereby fruits as provision for you ...
you may read:
The word "made" (ja 'ala) can mean "create" as is used in the ayat "He who created/appointed (ja'ala) the darkness and the light." (6: 1) and can also mean "designate" as in "We have made (ja'alna) it an Arabic Qur'an" (43:3) and "They have assigned (ja 'alii) to Him a portion" ( 43: 15)
Imam az-Zarkashi الزمخشري in his al-Kashaf تفسير الكشاف -see here in Arabic- made a statement about the major difference between the two verbs saying:
والفرق بين الخلق والجعل : أن " الخلق " فيه معنى التقدير ، وفي الجعل معنى التضمين ; كإنشاء شيء من شيء ، أو تصيير شيء شيئا ، أو نقله من مكان إلى مكان ;
The difference between (the verbs) khalaqa and ja'ala is that khalaqa includes a meaning of ability (being able to create) while ja'ala includes the meaning of inclusion, like creating onething from another, or forming one thing from another, or moving/carrying it from one place to another.
ومن ذلك : وجعل منها زوجها [الأعراف : 189] وجعل الظلمات والنور ; لأن الظلمات من الأجرام المتكاثفة ، والنور من النار وخلقناكم أزواجا [النبأ : 8] أجعل الآلهة إلها واحدا .
Examples of that are: "...and created from it its mate ..."(7:189), and (in this verse) "... made the darkness and the light ..." because the darkness comes from concentrated celestial bodies, while light comes from fire, "And We created you in pairs" (78:8) and "Has he made the gods [only] one God? ..."(38:5)
Sheikh at-Taher ibn 'Ashur in his at-Tahrir wa Tanwir التحرير والتنوير -see here in Arabic- added another difference saying:
، وفي الجعل ملاحظة معنى الانتساب ، يعني كون المجعول مخلوقا لأجل غيره أو منتسبا إلى غيره ، فيعرف المنتسب إليه بمعونة المقام . فالظلمات والنور لما كانا عرضين كان خلقهما تكوينا لتكيف موجودات السماوات والأرض بهما .
... And the "al-J'al" includes the meaning of association/affiliation, this mean that the object which was made actually was created for another or related/associated to another, so one may know the associated object by the connection. As darkness and light were "visible symptoms" their creation was in order to make an adaptation of the existing objects (in/on/of/the) heavens and earth by both of them.
So darkness and light (a-Dholomat and an-Noor) are a created to make for example the day on earth bright and the night dark. Or to enlighten a (part of a) planet by the reflection of the sun light or leave it dark (if we refer to as-Samawat as universe/heavens etc.).
