You can't backtest multi currency EA.
There is a creative workaround.
See my thread from a few moments ago:

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Hi, I coded an EA that can open positions on multiple symbols. So if I backtest it on GBPUSD I'm expecting it to open positions on other symbols as well, like EURUSD, but this gives an error. Here's the code that returns an error when I backtest in a different symbol than EURUSD. The error is error 4106 - "unknown symbol name EURUSD for OrderSend function"
Is this even possible? If not I don't understand why OrderSend allows to put any symbol in it...