You cannot have one child channel (same label) assigned to two parent channels. However, you can have one repository in sync with two child channels.
If you want to achieve something such as what you mentioned:
- Create both the parent channels, CentOS6_x86_64, and ScientificLinux_x86-64
- Create first child channel with label epel_6_x86_64, and choose CentOS6_x86_64 as your parent channel.
- Create second child channel with label epel_el_6_x86_64, and choose ScientificLinux_x86_64 as your parent channel.
- Create a repo named epel6_repo ( I assume you already did that from manage repositories tab)
- Now, go to each of the child channels' page, select repositories tab, select the same reposotory epel6_repo for both the channels, update repositories, and sync.
You'll achieve what you wanted. Here, we are accessing the same repo from two parent channels, with different names.