Sigasi HDT (and Eclipse) allow to perform multiple, simultaneous searches. The user interface is a bit hidden —this is considered advanced usage— therefore this short HOWTO:

If you do not want the search view to change and start a new search in a new Search view, you should pin the Search view. You can toggle the pinned state of the Search view in with the pin button on the right side of the Search view's toolbar.

Pinned Search View

When you pin the search view, the pinned search view will remain unchanged and subsequent searches will be shown in a second Search view.

This works for all search actions (VHDL, text, file, ...)