Hi Tomasz,
For the speed of your interconnect, check for this message in your errorlog when starting the second instance.
>> Average round trip time from instance 2 to instance 1 measured via 100 messages is 406 usec
Your data should only be accessed from a single instance. When you access a row (for row-locked tables), a lock is created on that instance and a shadow lock is created on the other instance. The datapage will not be in the cache on the other instance. As long as you don't access the datapage from the other instance, you won't have pages being transferred across the interconnect.
Regards,
Mark Kusma