I have two el-cheapo double pole systems. The first one is rigged with aluminum poles and the second one is with carbon poles. I rigged both systems by re-using aluminum booms.
The aluminum pole version is rigged with a swivel block simply tied to the single pole Spiro. I used this type of arrangement at the Adelaide Worlds on Earl Alexander's boat and it worked fine despite the high load on the cleat. One disadvantage is that you do lose a little effective pole length on the starboard side.
The carbon pole system uses Holger Jess tapered poles attached to another aluminum boom with the new Double Spinno from Warwick in South Africa. I believe Waterat makes a less expensive version as well. All you need to do with your boom is run a new shock cord 4x through the boom and exit aft via a through deck block. The poles were a little short so I made longer end fittings with Delrin stock from McMaster-Carr.
One issue we have been having with the Bixby lazy guy system is ring twist. To get past this we have changed our lazy guys to a stiffer line, Aricom-T. We have also flipped the poles around so the tapered end is inboard to cut down on pole bend on windy days