The Liberal Unionists fought elections against the Liberals. Did Liberal Unionists ever stand in the same seats as Conservatives too, or did they have a pact right from the beginning?
Yes. There was at least one case from the very start, during the 1886 election. This was of course the birth year of the Liberal Unionists, and they formed a close alliance with the Conservatives thereafter.
Despite the Conservative Chief Whip's promise, three of the 93 were opposed by Conservatives, and two lost their seats to Conservatives. Those two included the one Liberal Unionist who was opposed by a Gladstonian Liberal as well, who finished bottom of the poll.
Douglas, Roy. Liberals: A History of the Liberal and Liberal Democratic Parties. London: Hambledon and London, 2005.
There was kind of a pact from the very start. When the Liberal Prime Minister William Ewart Gladstone attempted to pass the First Home Rule Bill, the Conservatives sough support from within Liberal ranks. Liberal members who voted against the Second Reading of the bill were promised non-competition, and 93 of them sided with the Conservatives in the vote.