Hi,
The R100R has a single silencer. I know that Keihan http://www.keihan.co.uk/catalogue.html# used to make a 2 silencer system for the Paralever GS.
Try them,they looked quite smart too.
I think I'm right in saying that the issue with fitting other model 38mm exhausts is the side stand and exhaust clash.
I've always fancied a twin silencer set on my 100R-keep me posted.