Hey there. The Rowe boxes are quite particular about extension speakers. An improper load can blow the output devices, so be mindful of this. I'd recommend referencing a manual, because that has an entire section about attaching extension speakers and any deviation that you want to make from what I repeat from the manual below will also be following Rowe's specs.
Here's a conservative setup that would give you stereophonic sound from your extension speakers: According to my r-94 manual (same sound specs), using taps E1 (black) and E5 left (violet) and E5 right (pink) will dedicate 28 watts of the amplifier's power to the speakers in the jukebox. You could then attach your left extension speaker to left E1 and E5 and your right extension speaker to right E1 and E5. This will dedicate 14 watts for each extension speaker (combined 28 watts), which is nicely balanced with what the speakers inside the jukebox are drawing from the amp. Wiring the extension speakers will be as follows then: Left channel negative goes to left E5 and positive goes to left E1. Right channel negative goes to right E1 and right channel positive goes to right E5. The left channel and right channel extension speakers' polarity are reversed when compared to each other because the output phase has already been reversed on the output transformer assembly by design.
Polarity is important on this, so follow the hookups above closely, and the extension speakers should produce the desired sound level relative to the sound level of the speakers on the jukebox, hence the equal amount of wattage for everything.