Ideally you would want to connect the Speakon cable to both output channels of the power amplifier, which the sub would then sum together. However, since the power amplifier of the A5 is built in to the left speaker, there is no means of accessing the left channel amplifier output.
An additional constraint is that the only line-level inputs provided by the sub are an LFE input and a single-channel input for a full-range signal.
So assuming that the sub's LFE input would not be a suitable choice for use with your system, there only two possible ways that occur to me to get both channels into the sub. The first would be to get a mixer that would combine line-level L & R signals, and input them to the sub's single line-level input channel. The second is that if you are using a preamplifier or other upstream component which provides two pairs of INDIVIDUALLY BUFFERED volume controlled output jacks, you could combine the two channels of one of those pairs with a y-adapter.
Assuming that none of those approaches are viable, you would have to use the sub with only one channel being provided to it. That is not ideal, but may provide good enough results for your purposes.
To do that, connect both the yellow and red Speakon wires to the + (red) terminal of either speaker, and the black Speakon wire to the - (black) terminal of that same speaker. Or, alternatively, just connect EITHER the yellow or red wire to the + terminal of either speaker, and the black wire to the - terminal of that speaker, and don't connect the other wire. If you do it that way, the end of the unconnected wire should be covered with electrical tape, to prevent it from coming in contact with anything. Connecting all of the wires is preferable, though, as it would have less susceptibility to the possibility of noise and hum problems.