Replace both glyphs in calt

[FlorianPircher]

Use the following code in a feature like clig:

ignore sub @AllLetters R' o';
sub R' o' by R_o;

See here for more details:

Advice for Discretionary Ligatures?

You can use the following rule: ignore sub e' slash' o' @AllLetters; sub e' slash' o' by e_slash_o; Here, no space glyph is needed (which – as you said – can be problematic). Instead, the substitution rule is preceded by an ignore rule. The ignore rule says that the pattern e' slash' o' should not be matched if followed by a letter (@AllLetters). Glyphs can generate the @AllLetters class automatically for you. Click the plus + button next to the Classes heading in the Features tab and select A…