To add to the previously accepted answer, while the × or ×'s in anime titles will usually not be pronounced, leading to the idea that this is just for style or preference, this is not always the case.

Another reason I see as to why this is just for style or preference in some cases is because as far as I know, there is no × or X in hiragana or katakana, unlike in the English alphabet. I also have not yet seen an × or X in kanji or a representation of it in kanji. Examples of where the × is not included in the translation include (I got their translations from their respective Wikipedia pages):

• Hunter × Hunter - ハンター×ハンター Hantā Hantā, abbreviated: HxH
• High School D×D - ハイスクールD×D Haisukūru Dī Dī
• ×××Holic - ×××ホリック Horikku, pronounced as "Holic"
• Kiss×sis - キス×シス Kisu×shisu
• Servant × Service - サーバント×サービス Sābanto × Sābisu
• Dusk Maiden of Amnesia - 黄昏乙女×アムネジア Tasogare Otome × Amunejia
• Hybrid × Heart Magias Academy Ataraxia - 魔装学園Hハイブリッド×Hハート Masō Gakuen Haiburiddo Hāto

As you can see, the × is not included in the translation. If it was, it can be translated as 'ekusu' (エクス) or 'ekkusu' (エックス) (I'm still learning Japanese so any corrections on its katakana equivalent would be welcome). But, in some cases, the × might represent another word. I found an example in the anime Romeo × Juliet. According to its Wikipedia page, this is not pronounced as 'Romeo Juliet' but rather 'Romeo and Juliet' (ロミオ×ジュリエット or Romio to Jurietto) where 'to' is the Japanese equivalent of 'and'.

Thus, this varies depending on the anime or the manga title. I suggest you check the Wikipedia pages or something similar of works with the symbol '×' if you want to verify whether or not they are pronounced or if they are read as other words, like in the case of Romeo × Juliet, or if they represent something else.

In the case of Hunter × Hunter, it is not pronounced and has no meaning whatsoever. The image below is from Volume 6 of the VIZ translations.

Togashi was only focusing on naming a manga that has the format '(something) Hunter' and after seeing a joke about repetition, decides to repeat the word Hunter as a title. He did not mention anything relevant about including × so in my assumption, this has no meaning whatsoever, unless of course he only did that to prevent spoilers about something in his story that has yet to be revealed.

A warning if you are planning to search on some of the titles listed below as some contain NSFW content.

To add to the previously accepted answer, while the × or ×'s in anime titles will usually not be pronounced, leading to the idea that this is just for style or preference, this is not always the case.

Another reason I see as to why this is just for style or preference in some cases is because as far as I know, there is no × or X in hiragana or katakana, unlike in the English alphabet. I also have not yet seen an × or X in kanji or a representation of it in kanji. Examples of where the × is not included in the translation include (I got their translations from their respective Wikipedia pages):

• Hunter × Hunter - ハンター×ハンター Hantā Hantā, abbreviated: HxH
• High School D×D - ハイスクールD×D Haisukūru Dī Dī
• ×××Holic - ×××ホリック Horikku, pronounced as "Holic"
• Kiss×sis - キス×シス Kisu×shisu
• Servant × Service - サーバント×サービス Sābanto × Sābisu
• Dusk Maiden of Amnesia - 黄昏乙女×アムネジア Tasogare Otome × Amunejia
• Hybrid × Heart Magias Academy Ataraxia - 魔装学園Hハイブリッド×Hハート Masō Gakuen Haiburiddo Hāto

As you can see, the × is not included in the translation. If it was, it can be translated as 'ekusu' (エクス) or 'ekkusu' (エックス) (I'm still learning Japanese so any corrections on its katakana equivalent would be welcome). But, in some cases, the × might represent another word. I found an example in the anime Romeo × Juliet. According to its Wikipedia page, this is not pronounced as 'Romeo Juliet' but rather 'Romeo and Juliet' (ロミオ×ジュリエット or Romio to Jurietto) where 'to' is the Japanese equivalent of 'and'.

Thus, this varies depending on the anime or the manga title. I suggest you check the Wikipedia pages or something similar of works with the symbol '×' if you want to verify whether or not they are pronounced or if they are read as other words, like in the case of Romeo × Juliet, or if they represent something else.

In the case of Hunter × Hunter, it is not pronounced and has no meaning whatsoever. The image below is from Volume 6 of the VIZ translations.

Togashi was only focusing on naming a manga that has the format '(something) Hunter' and after seeing a joke about repetition, decides to repeat the word Hunter as a title. He did not mention anything relevant about including × so in my assumption, this has no meaning whatsoever, unless of course he only did that to prevent spoilers about something in his story that has yet to be revealed.