Why don't you just look at the entry requirements for the MSc Economics courses at the unis you think you might be interested in applying to in the future?
As far as I know, most MSc Economics courses and most economist graduate schemes require at least 50% of your modules to be in economics. So joint honours is generally fine, you may want to skew your optional modules slightly towards economics rather than history though if you want to do an MSc Economics. Equally, you could just do a MSc Economic History as well. Only concern is that you might not have the quantitative experience that some of the very top elite MSc Economics programs apply, but this very much depends on the uni that you'll be studying the BA Econ+Hist at, and where you want to study the MSc Economics