Hi Bratters, I've only got another 4 days to wait to get my hands on RB. My intention, next spring, is to erect several antennae on my roof (about 200' asl, with virtually 360 degree horizon, only 2 miles from Goohilly ES Station, built on The Lizard because of its flat, level and stable geological nature). I use an external sloping longwire for HF, but have a discone and Dressler ARA1500 which I need to get outside, and also plan purchasing Diamond 777 and a couple of other proprietary antennae, to get in the air, with an SSE1090, will also be buying a pair of JIM M-75s, and possibly a mast-head amp.. The plan is to keep downleads really short, until they're just inside the loft, easily accessible, where they'll be terminated with N or BNC type connectors, easily swappable/connectable singly or in multiple to JIMs or extension downleads. Once I've got the twigs up (more tackle than a Russian trawler!), I'll join the network and contribute to it.
With your situation, if you have 2x 1090 antennae, as long as they're more than 27cms (Wavelength of 1090 MHz) apart, I don't think you'll get any interaction or null phasing of signal if the signals are combined - I may well be wrong - but you certainly won't risk damaging the RB. If you were running a couple of active antennae, which receive their 12v via the downlead, you may run into problems.
I'll be trying the RB on several different combinations and see which gives the best results.