Suppose I'm generating EURUSD strategies and using a DataHorizon of 1 year.

And then I include Multi-Market testing and require that my strategies pass 3 additional symbols -- e.g. USDCHF, GBPUSD and USDJPY.

My question is then -- how is that different than using a DataHorizon of 4 years and not including Multi-Market testing?  I mean, when you include Multi-Market testing isn't it roughly equivalent to increasing the DataHorizon?  Or are the signal patterns from different symbols significantly different that Multi-Market testing stress-tests your strategies in ways that increasing the DataHorizon can not?

Thanks for any insight you may have...