Best way to join two large datasets in Pandas

I'm downloading two datasets from two different databases that need to be joined. Each of them separately is around 500MB when I store them as CSV. Separately the fit into the memory but when I load both I sometimes get a memory error. I definitely get into trouble when I try to merge them with pandas.

What is the best way to do an outer join on them so that I don't get a memory error? I don't have any database servers at hand but I can install any kind of open source software on my computer if that helps. Ideally I would still like to solve it in pandas only but not sure if this is possible at all.

To clarify: with merging I mean an outer join. Each table has two row: product and version. I want to check which products and versions are in the left table only, right table only and both tables. That I do with a

pd.merge(df1,df2,left_on=['product','version'],right_on=['product','version'], how='outer')