One asynchronous operation per context is permitted at a time, as is made apparent by the exception.
So, you must choose between
as the error notice advises, one at a time:
var banner = await context.Banners.ToListAsync(); var newsGroup = await context.NewsGroups.ToListAsync();
Or you might employ several contexts:
var banner = context1.Banners.ToListAsync(); var newsGroup = context2.NewsGroups.ToListAsync(); await Task.WhenAll(banner, newsGroup);
Consider using the "transient" or "PerWebRequest" type for your lifecycle if you are using an IoC container for the injection of your Data Provider.