Hi Wolf,
In principle, yes. You'll just have to check whether the 10-20 doesn't vignette at 10 mm and infinity focus with a standard thickness pola filter if it isn't a slim version. It will likely work, however.
Just try shooting a well illuminated wall, from any distance, with the lens set to 10 mm and focus on infinity, and the filter mounted of course <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' />. If you only have a gradual light fall-off towards the corners, you should be fine.
HTH, kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....
Get a good multicoated one, like for instance a B&W. Your probably do not need a slim one, a normal profile B&W should do fine too on the Sigma 10-20mm. But of course, that is quickly found out in shop.
Contrary what is being said above, you can of course well get a very dark sky with an UWA lens. Only thing is that the sky will not be as even of shade as with a lesser viewing angle, that is the nature of polarized light distribution.
So anyway, yes, if both have the same thread size, both can use the same pol. filter.
Thank you all. It seems a slim good quality will do the job. I know I can get the Hoya in time for Jan 1 departure, but the Marumi I don't know. I'll let you all know how it worked out. Caribbean sun will justify the use and hopefully the results will be OK! Kind rgds Vieux Loup