material design – Is it a bad practice to have a two line title in header?

It is a good question and I think it doesn’t have a clear answer. If I understand correctly the context, the title of the page is “Transactions”. The account number is actually the parent of this page.

Although you are not asking for Material design specifically I suggest you take a look to Structure app bar. In the bottom of the page there are some examples that make the Top bar double height so they can use the lower part as breadcrumb. I believe this might be a useful way of showing the hierarchy in your case.

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The approach you have right now is clear, but using a double line in a short toolbar makes the line height too small and thus the text a bit cluttered.