Image Blender is a great iPhoneography app allowing you to blend images together with blending modes including Multiply, Overlay, Screen and more. This leads in turn to allowing you to create your own personalized filters as well as allowing you to create double exposures with control.
This update includes bug fixes and speed improvements as well as a single tap on preview for editing.
This is a free update but if you haven’t already purchased this app you can do so here. It retails for $1.99/£1.49/download.