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Can I have multiple rows of tabs?

Yep this can be achieved with css, have a play.

What about multiple sets of tabs, like one at the top of the page and one at the bottom?

Yes and No. You can use dtab_list_tabs as many times as you want (setting "tabbar=class" as a parameter to ensure valid css) to procude several copies of the same set of tabs, but currently you can only set up and administer that single set of tabs. Multiple tab sets is a feature I am considering adding in future versions.