Thank you all for the answers so far. I will try it and thought about it all day.
Currently, I have an antenna on the North edge for GNSS rx. And another different antenna on the West edge for an ISM tx/rx around 868MHz.
Now, could I replace these by two identical dual-band ISM+GNSS linear antennas, link them to the coupler and use its RHCP path for GNSS and its LHCP path for ISM ? Would these indeed be orthogonal and naturally quite isolated from each other ?
The GNSS path contains a SAW already.
The objects are handheld and nothing guarantees their orientation so circular polarization on the ISM link could be better. Line-of-sight is guaranteed.
Thank you very much ! This is exciting.