Can I have my Garmin GTX-345 and Flight Stream 210 connected to ForeFlight at the same time via Bluetooth?

No. The installation of a Garmin GTX-345 and a Flight Stream 210 requires the GTX-345 Bluetooth to be disabled and the GTX-345 be physically wired to the Flight Stream 210 so that the ADS-B weather and traffic data is sent to ForeFlight Mobile only through the Flight Stream 210's Bluetooth connection. Your avionics shop does this as part of the installation process.