We cover our bikes with a double 'bike pyjamas' when touring, so it looks like we have a 2nd small tent, it's made out of woven plastic and keeps the biks dry and warm (and also out of sight). The Pyjamas also double up as a picnic blanket and on one memorable occasion an emergency tent when caught out in a summer deluge
We lock them together using a 2m kyriptonite cable to a dog spike
Spike, Cable and Pyjamas weighs in at about 2kg and lock can be same again, but that is optional. Spread over two bikes is minimal exta weight and give a lot of peace of mind