I don't have any experience with the USB Sound Card you are using but there is no reason why you couldn't simply feed both of the speakers (amplifiers) from the same output using a splitter.
Alternatively, you could use a fairly cheap DAC Hat which has both RCA and 3.5mm (headphone) Jack on the RPi. The HifiBerry DAC + and the IQAudio DAC Pro have these outputs as does the AliExpress PiFi v2 https://www.aliexpress.com/item/32985575...1112%22%7D
While none of these solutions are optimal, at least they will not suffer from the asunchronous clocks problem discussed earlier in this thread.
Alternatively, you could use a fairly cheap DAC Hat which has both RCA and 3.5mm (headphone) Jack on the RPi. The HifiBerry DAC + and the IQAudio DAC Pro have these outputs as does the AliExpress PiFi v2 https://www.aliexpress.com/item/32985575...1112%22%7D
While none of these solutions are optimal, at least they will not suffer from the asunchronous clocks problem discussed earlier in this thread.
