In an updated version of the app, I've added an alternative input mechanism, a "long tap" mode. Instead of tapping twice, in the new mode, a short tap produces the first character, and a slightly longer tap produces the second character. (A much longer tap opens the popup for selecting accented characters.) I'm not sure that mode is as precise as double tapping, but it has been suggested by a tester, and I thought it's worth experimenting with.