Is there any way possible (other than by hand) to backtest across different assets? For example, let's say I only wanted to trade a certain Forex pair on days that the S&P did something specific. Or what about using different time frames? Such as using the same pair, but consulting data from the daily chart to make decisions of when to trade on the M15 chart?
That is easy with any backtesting solution that supports multi-symbol strategies. It is trivial with https://quantterminal.com/ as it should be with any commercially viable solution.