The single/plural duality of 親 as a concept is actually not that different from 子供, in that these phrases parallel each other:
I was thinking about this further, and noticed one good reason that 親たち is so infrequently used.
It is rude
親 is simply rude when referring actual parents, and not ...
I agree with you that たち is normally used with animate objects. One obvious exception for this rule is when explicit personification is involved. For example, sentences like 山たちが私に語りかけてきた, 乱暴に扱われた本たちが悲しんでいる are perfectly fine.
In your question, I think this アプリたち doesn't look that weird to me. Of course I'm not saying アプリたち is a kind of personification, but ...
There's no firm distinction between たち and ら at least in modern spoken Japanese except that the latter may sound old-fashioned.
For the particular quote above, it sounds a bit like a translation from English, as is common with tech curated blogs. But it's still within natural and common Japanese of today.
To answer the specific question: No, there is no restriction. You can use 〜たち with two people but you aren't exactly saying "two people", you are saying "groups of ~"; the definition given by JDIC being pluralizing suffix. 〜たち is used to refer a group and this group could very well include only two "animate objects" (or groups ...
In this case, I think it's simply because we already have a specific and common word which refers to "one's both parents": 両親【りょうしん】. There is no reason not to use 両親 in formal situations.
I feel I sometimes hear (私の)親たちは旅行中だよ (instead of 両親は旅行中だよ) in casual conversations.
Japanese language doesn't have plural form of noun like English. So we can't know how many children are playing in the park in this sentence "子供が公園で遊んでいます".
If you want to say "A child is playing in the park", you say "一人の子供が公園で遊んでいます。"
If you want to say "Children are playing in the park", you say "子供たちが公園で遊んでいます。".
Your translation is fine. Did you notice that 同士 is actually "reflected" in your English? I mean:
同士 might be a tricky word, but not totally untranslatable to English if you bear with a disproportionately lengthy definition, that X同士 means those each of who/which is (equally) a X to each other. And this may explain why it is usually "not ...
It is the formal (or polite) way of conjugating verbs.
書く (kaku) write (dictionary form)
書きます (kakimasu) write
書きました (kakimashita) wrote
書きません (kakimasen) don't write
書きませんでした (kakimasen deshita) didn't write
私は本を書きます。 (watashi wa hon o kakimasu)
I write a book.
です (desu) is
でした (deshita) was
ではありません (dewa arimasen) is not
ではありませんでした (dewa arimasen ...