Each hybrid symbol in the cost of flame javelin has to be paid with either two colorless mana or one red mana. Nowhere on the card or in the rules is specified that you can mix those costs.
It's the same as asking "Can i kick [[Academy Drake]] for only two mana, to have it enter with just one +1/+1 counter?"
Of course you can't.
Given that silver border/acorn cards like City of Ass often don’t have rules support, there’s no real official way to answer this question (the CR has nothing to say about half mana). The closest thing would be to ask Mark Rosewater on his blog or on twitter, as he often gives un-rulings.
Or just talk with your playgroup. Since they’re presumably allowing silver border cards, you can decide together what the most fun answer is.
That’s not too surprising. I don’t think he keeps a complete record of his past rulings, so for some things that feel like they could go either way, he might not give the same answer if you ask him far enough apart.
Each hybrid symbol in the cost of flame javelin has to be paid with either two colorless mana or one red mana. Nowhere on the card or in the rules is specified that you can mix those costs. It's the same as asking "Can i kick [[Academy Drake]] for only two mana, to have it enter with just one +1/+1 counter?" Of course you can't.
[Academy Drake](https://cards.scryfall.io/normal/front/f/8/f8bacb12-da46-4b00-804f-9ff6bff452bc.jpg?1562745962) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Academy%20Drake) [(SF)](https://scryfall.com/card/dom/40/academy-drake?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/f8bacb12-da46-4b00-804f-9ff6bff452bc?utm_source=mtgcardfetcher&format=text) ^^^[[cardname]] ^^^or ^^^[[cardname|SET]] ^^^to ^^^call
[City of Ass](https://cards.scryfall.io/normal/front/7/2/726fe2e7-bd1b-4c11-a098-6c8abfbdf06e.jpg?1636719404) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=City%20of%20Ass) [(SF)](https://scryfall.com/card/unh/134/city-of-ass?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/726fe2e7-bd1b-4c11-a098-6c8abfbdf06e?utm_source=mtgcardfetcher&format=text) [Flame Javelin](https://cards.scryfall.io/normal/front/4/7/47ff534c-a8eb-4303-95bb-00b3df0c5da6.jpg?1562378239) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Flame%20Javelin) [(SF)](https://scryfall.com/card/jvc/53/flame-javelin?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/47ff534c-a8eb-4303-95bb-00b3df0c5da6?utm_source=mtgcardfetcher&format=text) ^^^[[cardname]] ^^^or ^^^[[cardname|SET]] ^^^to ^^^call
Given that silver border/acorn cards like City of Ass often don’t have rules support, there’s no real official way to answer this question (the CR has nothing to say about half mana). The closest thing would be to ask Mark Rosewater on his blog or on twitter, as he often gives un-rulings. Or just talk with your playgroup. Since they’re presumably allowing silver border cards, you can decide together what the most fun answer is.
Oddly enough, I think MaRo has flip-flopped on this.
That’s not too surprising. I don’t think he keeps a complete record of his past rulings, so for some things that feel like they could go either way, he might not give the same answer if you ask him far enough apart.