Both tweeters are supposed to radiate. Here is a schematic of the 2.5s
http://www.infinity-classics.de/technik/manuals/RS_2.5_technical_sheet.pdf
Loosen the front tweeter mounting screws and make sure the wiring isn’t loose. If it’s not that, check the disconnected tweeter with an ohmmeter - it should be between 3.2 and 3.8 ohms. If it is open, it is dead. If you need a new one, replacement tweeters are fairly plentiful on eBay.
http://www.infinity-classics.de/technik/manuals/RS_2.5_technical_sheet.pdf
Loosen the front tweeter mounting screws and make sure the wiring isn’t loose. If it’s not that, check the disconnected tweeter with an ohmmeter - it should be between 3.2 and 3.8 ohms. If it is open, it is dead. If you need a new one, replacement tweeters are fairly plentiful on eBay.