Were you ever successful big-banging a second i2c bus on the RPi 3? I've seen different sites showing a 'dtoverlay', but it doesn't seem to work for me. I see a result of a "i2cdetect -y 3" but very very slow, and no devices show up. I've got a (probably over-large) project for a RPi 3 and need the speed of two i2c buses. If you have any suggestions, please reply here. Thanks!! BTW: I'm using a RPi3 and Stretch.