history - Why was both katakana and hiragana created?

Nowadays, katakana tends to be used for gairaigo and onomatopoeia, while hiragana tends to be used for native Japanese words. This is a slight simplification - more information is available here.

However, English loanwords weren't a major concern when they were originally created. What were the main motivations for creating the two kana (or possibly more?) rather than just one?

Answer

It isn't 100 percent clear, but the following is the “well-established” theory:

As noted in your other question, hiragana was originally called 女手{おんなで}. In the late Nara, early Heian periods, 万葉仮名{まんようがな} written in 草書体 (sosho style) was used for “unofficial” texts such as Japanese poems (和歌{わか}), etc. From this 万葉仮名, women in the imperial courts (宮中の女官達) developed the simplified writing style (女手 or what is now called “hiragana”). Originally, it was mainly used by women (hence the name 女手), but because of its ease of use compared to Chinese characters, men also began using it when writing. However, “official” writing still used Chinese characters and hiragana was used mainly among commoners (non-governmental people) and in poems and short stories, etc. Also, the 平 of 平仮名 also comes from its “simplicity” and “general use”. You can see it in words like 平易、平凡、平素, etc.

Now, katakana also originates from 万葉仮名, however it came into use first by Buddhist monks. Also, unlike hiragana, it was not used as a separate writing system from Chinese characters, but rather together with Chinese characters. In order to read those difficult Buddhist scriptures (お経), Buddhist monks would use katakana as a form of shorthand or annotation (called 訓点) as a supplement to the Chinese characters. In this way, katakana became used in official documents and for scholars, as Buddhism, 学問 and government was very closely related. Since it mainly was used by men, some people referred to Chinese characters and katakana as 男手 in contrast with hiragana (女手). Also, the 片 of 片仮名 comes from the fact that “pieces” of Chinese characters were taken to develop the writing system (and also there is implication that katakana is “temporary” in the sense that it is only to supplement the Chinese characters).

So to answer your question, the reason that the two exist is because they were created separately out of need at the time and they were originally used for two different purposes ― hiragana for a common language separate from the official writings (which used Chinese characters) and katakana as a supplement to official writing (and other text that used Chinese characters).