Using a single Airport Express would require way too long of a line out cable going to the other side of the room, and I don't know how the Audioengine's inputs would react to being paralleled with a Y-cable as well.
The solution would be to use two Airport Express, one at each speaker connected with the included 6" line cable.
Airport Express can be regularly sourced new or like new on eBay for ~$50.
Note: I assume you are going to send stereo to each pair of speakers. I mention this because the current AE wireless 'N' version will distort badly if the line output is passively summed to mono, whereas the previous AE wireless 'G' version worked fine in that mode.
Steve