Having two search submit buttons is bad for site usability. Users end up confused both what button to click and what the heck the difference between them is.
This actually got us a place in a list of things NOT to do:
This is basically why simplesearch was introduced in vector.
For other skins however we actually have $wgUseTwoButtonsSearchForm. Which when set to false replaces the unusable "Go" + "Search" with a single "Go" button and an "Advanced search" link that links to the search page and acts as a usable replacement for the fulltext search button.
- Deprecated in 1.35
- Removed in 1.36