From the Siege FAQ there is nothing to conclude it has a maximum of concurrent users, it does offer some advice, but not on increasing the load.
Keep in mind that -c256 -d1 translates to roughly 47,000 unique
visitors per day.
Each physical machine will have a certain limit based on CPU, Memory and number of network sockets. Technically a single machine could handle 10K+ connections, in this question talks about 1,6 million simultaneous sockets connections on a pretty heavy machine.
Opening more terminals will probably not let you increase the number of connections on a single machines. You could just increase the
-c number, until the machine gives an "Error: system resources exhausted" error or something similar. Experiment with what your machine can handle. Alternatively just try it and find out or ask the same question to the developers at GitHub-issues.
If you need more load with Siege I think you should just get more physical or cloud-machines to run it on, just start some scripts at the same time. Have a look at clustershh to send the same command to multiple machines at the same time if timing is important for you.