How to combine channels of different length based on a shared key

[ChriKub]

I am stuck trying to combine two channels of different length based on a shared key. The two channels should be combined in a way that the resulting channel contains all entries from the bigger channel with the fitting entries of the smaller channel.
The two channels look like this:

small channel:
[chr1, fileA, fileB], [chr2, fileX, fileY],...

large channel:
[chr1, chunk1.1], [chr1, chunk1.2], [chr1, chunk1.3], [chr2, chunk2.1], [chr2, chunk2.2], [chr2, chunk2.3],...

The resulting channel is supposed to look like this:

[chr1, fileA, fileB, chunk1.1], [chr1, fileA, fileB, chunk1.2], [chr1, fileA, fileB, chunk1.3], [chr2, fileX, fileY, chunk2.1], [chr2, fileX, fileY, chunk2.2], [chr2, fileX, fileY, chunk2.3],...

I can't find a channel operator in the documentation that does this. Any help is greatly appreciated!