Parts Express give free postage to the USA and there is only a discount if you buy 4 or more shakers at a time so it doesn't make any financial difference whether you get one now and another later or both now.
Without experimenting it is difficult to know how much effect you will get in a chair - a metal frame will transmit the vibration better than a wood one, so for instance I get a bigger effect with one shaker in each recliner than I get with four shakers in the centre, wooden, non-recliner part of the sofa.
You could start with one or two depending on preference, but I think you are going to like the effect so much that you will end up with two! It also depends on how much room / clearance you will have for mounting.
The effect is variable within games and from one game to another - GT5 is generally good, but varies from car to car - strong effect from Audi R8 V8 and all Mercedes Benz V8's, weak effect from most six cylinder cars. Very poor effect with Dirt 3. I haven't tried Forza, but have heard the tactile effect is good. So bearing in mind the variability I suppose it would be best, subject to finances and fitting space to get two shakers. If using two shakers wire them in series if using the Dayton amp to get an 8 ohm load.
You can mount the shakers with screws, but M4 bolts with nylok nuts and mudguard washers will give the best results. Use decent thickness speaker wire to prevent loss of signal - the standard home cinema stuff is ok for testing, but not for a permanent install.
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