The pair's arrival marks the latest expansion for the European arm of the US-headquarted ad optimisation company.
Their role is to boost publisher clients' CPM rates by matching traffic with the best advertising network partners as well as using targeting technologies including behavioural, geographic and contextual targeting to increase their revenue.
They will report into Oli Whitten, director of ad network development for EMEA, who arrived at the company in September from AOL Advertising.
Both are former MySpace colleagues of Jay Stevens, a vice-president and the international general manager of The Rubicon Project.
Jakovljevic has four years' experience at a European level, including two years monetising unsold inventory for MySpace across EMA through advertising networks.
Siotis spent two years' experience monetising all non-guaranteed inventory across Fox Interactive Media sites including IGN, MySpace, AskMen.com and Rotten Tomatoes, online and on mobile. Prior to that role he worked at Advertising.com.