Yes they are. I have spoken to a supplier who seem to think this should be ok due to the fact that built in is a devise to retune in case you pick up a neighbours wireless headphones so I guess the ssame should apply to two of your own units hey?
I have experience with the RS65s, and they have 2 wireless channels at different frequencies, and iirc in the manual it mentions that you can connect two pairs to one base station (not what you want, but similar in execution), but I don't know if you do this by using 1 channel for both or two channels separately. However, it's been a while since I checked, so please don't take my word for it.
Looking at the sennheiser website regarding supplemental headphones for the RS65 base station:
" Additional RF wireless headphones (863 MHz) HDR 65-8 04964
Supplementary headphones for the RS 60 and RS 65 systems, available as 434 MHz, 863 MHz and 925 MHz version."
Well, there are apparently 3 channels. I guess the key thing is to ensure that your two sets of headphones can be used on two of them, and you should get no interference...
And this comment from the RS45 website implies it really is possible:
" Two switchable channels for simultaneous operation of several systems"