Just like in English: "Show up" is, ostensibly, two words, but it's a phrase with a different meaning that either "show" or "up". So yes, pretty much. But "Blue Sky" is two words indicating a single thing, but that thing is both "blue" and "sky", so no, you're not really expanding the language.
That said, I don't speak Tiki Pona, so I don't know how the language is used by the various communities that actually speak it.
This kind of phrase usually has a historical origin.
I tried to find out what it is for "show up" but apparently, this is a very recent one and originally meant something else entirely:
"verbal phrase, by 1826 as "to disgrace through exposure;"
It then changed to the current meaning only in around 1888, just ~135 years ago:
"The meaning "to put in an appearance, be (merely) present" is by 1888."
Unfortunately, I couldn't find any explanation based on historical use of the phrase (phrasal verb, to be accurate), but given it's a quite recent one, it probably originated from people just trying to emphasize "completion" of the "show" action by adding "up", like with most other phrasal verbs.
Thanks! Yes, I remember reading older novels (Austen, maybe?) where the "disgrace through exposure" sense was used... it was obvious what was meant, but surprising.
"disgrace through superior performance" is a current meaning. AA does a thing, and then (especially if AA appears to be getting praise for it or appears proud of it), BB either does it better or does something more difficult (usually accompanied by a derisive remark about AA or AA's work).
That said, I don't speak Tiki Pona, so I don't know how the language is used by the various communities that actually speak it.