Hi-ya Christian,
My 660Ti registers at ~270 khash/s peak (scrypt) for a single instance of cudaminer (which appears to start a single mining thread) - By starting two instances, I'd have expected that rate to halve. Instead, each instance reports ~200khash/s peak.
This is surprising. What is your launch configuration?
What does GPU-z show for GPU utilization when running just a single instance?
How's GPU utilization and memory usage with 1 and 2 instances?
I used the the same launch config (-l K7x32 -C 1 -i 0 -m 0 -H 1 a product of not realizing it was already running in another terminal). After messing around with it for the last day, I can tell you the reported numbers on the second cudaminer instance are lies.

Juggling the launch config around (7x32, and the other at 7x14), I can get two instances to submit results correctly, but they don't exceed my 270 khash/s (one settles at 180 something khash/s and the other at 80 something). There's a +500MiB GPU memory usage increase in this scenario (Don't know about utilisation... nvidia-smi won't report it). There doesn't appear to be any operational efficiency to gain here.
Apologies for wasting your time.
