Mixing double cream with single cream?!

Is it advisable? I'm basically a beginner to baking so not really sure myself. Today i saw a recipe that stated that you need 300mls of whipping cream, although all i have is 150mls of both double cream and single cream. I know it obviously won't turn out how its supposed to turn out but would mixing the double and single cream together provide a decent outcome? The cream is to go on top of a banoffee pie.

Thanks