I too carry two spare tubes for each size wheel on my recumbent trike. Much faster than trying to find the source of the leak. They don't weigh much and fit nicely in underseat pouches.

You can try cleaning the tube with a hydrocarbon solvent to remove the residue from the temporary patch. Carb or brake cleaner, paint thinner, or Coleman fuel all work well if applied with a Q tip. Whenever I patch a tube with a regular patch, I clean the tube with solvent before applying the glue. Makes the surface squeaky clean.