Yes, David; that's the wiring shown in the diagram in the manual. As Alex mentioned above, however, amplifiers which are designed internally for bridging use one + post and one - post to send the two signals in opposite polarity, as required. The W5.3 obviously doesn't have the internal circuitry for this and uses an external bridging adapter(which certainly isn't just a straight Y-adapter)to create the needed polarity inversion. $350 is an outrageous price, which somehow doesn't really surprise me.
As I commented in my previous reply, there appears to be no good reason for you to do this.
