I installed a buttkicker in a two-seat reclining couch that sits on a riser (I built a false floor on the couch where I installed the buttkicker to ensure thorough transference throughout the couch because the frames of the two seats were largely disconnected). Having a buttkicker within the furniture provide all the shake you will want. There is no need at all for buttkickers in the couch AND on the riser.
Regardless of where you choose to install your buttkickers, the main thing you MUST do is use proper rubber isolators. If you install on the riser, the riser must rest on rubber isolators. If you install in your couch, your couch must rest on rubber isolators. It can't be just any rubber either. I tried a couple varieties before finally going with the isolators Buttkicker sells on their site. The ones Buttkicker sold me worked much, much better. Isolators enable accurate articulation and movement of whatever the buttkicker is attached to. Without the isolators, that movement is lost. I had to turn my buttkickers up to 75% power to get the same movement that 25% power provides with isolators. Not only that, but the movement with isolators is much more accurate and believable.
Each Buttkicker LFE comes with 5 of their smallest size rubber isolators, but these are usually insufficient for a couch because under too much weight they overcompress and fail to do their job. They may, however, work okay for each individual chair (if they are light enough). If those don't work, you can buy the next larger size of isolators, which are great. Don't do that, however, unless you can see the smaller size overcompressing. It was very noticeable on mine.
If you go the riser route, you'll need to buy much larger and more expensive rubber isolators for the riser to rest on. How many and where you put them will depend on the size of the riser and the weight of the objects it will support.