Need two multiple inputs for one stereo rca input

[aswerdlow5]

So I have a pair of Bose Companion 2 desktop speakers with two stereo RCA inputs (red and white), I have my computer, and chromecast connected, but also wanted to connect my amazon echo dot. I tried purchasing a dual male 3.5mm to single female 3.5mm adapter (I currently have two rca to 3.5mm male plugged into my 2 inputs on my speaker), but it turned out to be a mic + headphone to 4 pole jack adapter. Does anyone have any suggestions and preferably amazon linking with any solution? I tried finding stereo rca to dual 3.5mm male, but I could not find any. I'm not planning on playing them at the same time, so a passive adapter would be fine.

Thanks

(EDIT: Here is the link to my speakers for reference https://www.amazon.com/Bose-Companion-Multimedia-Speaker-System/dp/B000HZDF8W/ref=sr_1_2?ie=UTF8&qid=1480309740&sr=8-2&keywords=bose+companion+2)