Loop within a loop

I have a loop, getting all post titles in a category.

I then want to do a loop, within that loop, which gets all posts with a sub category of that category.

I be all like:

Title 1
— Sub cat Title 1
— Sub cat Title 2
— Sub cat Title 3

Title 2
— Sub cat Title 1
— Sub cat Title 2
— Sub cat Title 3

Title 3
— Sub cat Title 1
— Sub cat Title 2
— Sub cat Title 3

Hope this makes sense :S

Just, can I do a loop inside another loop? Lol.