Strathclyde Police held a joint press conference with Crimestoppers in Madrid to appeal for information on Derek McGraw Ferguson and William Paterson.
Paterson is wanted in connection with the murder of Kevin "Gerbil" Carroll on January 13, 2010, in an Asda car park in Glasgow.
Ferguson is alleged to have shot dead Thomas Cameron outside the Auchinairn Tavern in Glasgow on June 28, 2007.
Strathclyde officers will be working with Spanish colleagues in the south of the country to hunt for Ferguson and Paterson.
They will also be speaking to local people and business owners to hand out a Crimestoppers leaflet, printed in Spanish, with pictures of the two men and an appeal for information.
Speaking in Madrid, Chief Superintendent John Cuddihy, head of serious and organised crime and counter terrorism, said: "Strathclyde Police has an excellent working relation-ship with law enforcement in Spain and we will continue to work together to ensure that Spain will be a hostile environment for fugitives.
"We will do everything possible to trace both fugitives, bring them to justice and remove them from your country.
"There is no doubt in my mind that to evade police for this period of time both these men have been receiving assistance from associates who live in these areas.
"Officers are and will continue to be on the ground in Southern Spain in areas these two men are known to frequent, to give information to locals and business owners about these men and how they may well be living amongst hard working, honest people.
"I have no information that either of them pose any direct threat to the safety of the Spanish public, however I would urge the public not to approach them."
He urged anyone with information to contact Crimestoppers in Spain on 900 555 111 or in the UK on 0800 555 111.