As expected, if I manually write out the assets, I can get them to plot on the same chart.

Code
	asset("EUR/USD");
	vars Prices1 = series(price());
	var RSI_plot1 = RSI(Prices1,14);
	
	asset("EUR/CAD");
	vars Prices2 = series(price());
	var RSI_plot2 = RSI(Prices2,14);
	plot("EUR_RSI", RSI_plot1, LINE, RED);
	plot("EUR_RSI2", RSI_plot2, LINE, BLUE);
}