The Tandem adaptor is to be slotted into the first set of filter holder and then you clip the second set of the filter holder onto it. This allows independent rotation of grads, as well as the option to use more than four filters if desired.
However, this definitely will cause vignetting if you are using ultra wide angle.
It does not have any screw threads for screwing. Just to clarify.
Don't have a cross section diagram but the closest I can get is this picture as per the link.
Canon 5D Mark II + Canon EF 17-40 F/4 L USM + Lee Professional Kit | Flickr - Photo Sharing!
The configuration is
Lens -> Adaptor Ring -> First Filter Holder (Maybe 2 slots) -> Tandem Adaptor (Slot into the first filter holder last slot) -> Second Filter Holder (Maybe 2 slots)
So effectively, instead of having 4 slots but you only get to use 3 slots because one is used to hold the tandem adaptor. If you are using this configuration on a wide angle of DX 10mm to 12mm, I believe there will be severe vignetting.
Not sure Cathay carry them separately because this is very rare and Cathay only has the Professional Kit which include two filter holders and a tandem adaptor. Can try to call them.
If you want to use a polarizer with your filter holder set, then you can cosider 2 options.
Cheap yet troublesome method.
Get a thin Polarizer, rotate it to your desire angle, then use a gaffer tape to fix the position, then screw on the adaptor ring then clip on the filter holder and the rest of the filter.
Expensive method
Get the 105mm accessory ring, then get a 105mm polarizer from Lee or B+W. I personally tried it before. The B+W is slightly thinner than Lee but the polarizing effect by Lee may be better.
Based on your method, the configuration and use is as follows.
First Filter Holder with a a 100x100mm square polarizer filter piece, then using the tandem adaptor, clip on the second filter holder with another 100x100mm square filter.
However, as per Lee filter's website. The 100 x 100mm square polariser slots into the filter holder, which is then rotated to achieve the desired effect. This version is recommened for studio use, or when no graduated filters are required in addition to the polariser.
Lee Filters Polariser
So hope that answers your queries.
Good day.