Generally, in history it's preferred that the writer use footnotes.
More than one footnote should never be included side-by-side.
If you need to reference more than one source, use only one footnotes and include the bibliographic information for all the sources in the same note.
On one side of the citation spectrum you've got direct quotations-material typed word-for-word from the source text-that should always list the source.
On the other side of the spectrum you have your own personal arguments and ideas; obviously these won't have a source to cite. Often it can be difficult to tell where your research ends and your own ideas begin or whether a fact or idea can be considered common enough to skip the citation.
Other style guides such as MLA or APA will have different rules for when and how to use footnotes. Basically, it's a number inserted into the text that directs the reader's attention to another location in the paper where they can find more information about what they've just read.
In history papers footnotes serve as a way to cite sources, and the note is usually a bibliographic entry that details the source material for a quote or idea.
When you do need to cite the same source more than once, you can use a shortened version of the bibliographic entry.
If you're citing the same source in two footnotes in a row, you can use the abbreviation ibid with the page number.
When applicable, incorporate additional information in the main text of the paper, but avoid inserting irrelevant material.
Footnotes should briefly present the reader with meaningful information that enhances your argument.