Openlog Artemis: Serial1.AvailableForWrite always return 0

The SoftwareSerial is working in parallel to the hardware/UART. So yes you can do what you propose.

Another, maybe easier, test you can do is to write to serial1 one byte aa time from your sketch. So do not write the complete buffer from you sketch. By doing one byte at a time, it first checks and waits that a byte can be written to the UART module. without doing a lock. The lock is happening once it starts writting.