It is very difficult to find literature on this topic and the only related question is Splicing である with a topic whose answer is (to my view) a bit elusive and the reference provided by snailboat is way beyond my understanding.
So my question is when it is possible to split the two entities of である and what can be put in between ?
I really don't quite understand which part of the fantastic answer on the linked question you don't understand. I'll try to add some notes.
である is for all practical purposes a copula (like だ). This means in particular that your sentence structure will consist of a topic/subject and a predicating noun phrase.
The topics/subject may be marked by は/が/も; the predicating noun phrase is marked by である (for this example; generally だ, です, ...). Schematically,
A [は・が・も] B である。
You know that you can insert the topic/subject between で and ある for emphasis
B で A [は・が・も] ある
Note that this shortens the distance between the topic/subject and ある.
Other examples, which may be found on the internet
ご自身の責任で 完全に ある。
It's entirely your own responsibility. ???
are definitely understood, but certainly considered unnatural/incorrect.
So, in a word, you can split で ＋ ある for (strong) emphasis to insert the topic/subject.
(Just for completeness.) I guess you were asking about adding in more than a particle, because particles you can add in quite readily:
である ＋ も = でもある
である ＋ は = ではある
(Cf. でない ＋ は = ではない = じゃない & でありません ＋ は = ではありません = じゃありません)
Note that でもある is the only way to try to combine も and (the copula) だ:
これは赤ペンだ。黒いペンでもある。
This is a red pen. It's also a black pen.
