Idea

When the multiple -c options are given like this

$ xpanes -c 'echo 1@{}' -c 'echo 2@{}' hoge 

All the arguments of -c options are combined with new lines and result is like this.

$ echo 1@hoge
1@hoge
$ echo 2@hoge
2@hoge

Why this is helpful?

I think some users hope to change the default behavior.
The change what the user hopes can be implemented with use's customize command line like #84 .
If the multiple -c options are allowed, users can define alias of xpanes very easily, like this.

alias xpanes="xpanes -d -c original_command_always_first"

Creating alias of xpanes command is helpful idea (like #64).
And this feature allows users customize tmux-xpanes's behavior more easily and flexibly.