Is it safe to mix methylated spirits with isopropyl alcohol?
Yes, as far chemical safety is the concern. No dangerous reactions nor dangerous phenomena will occur. The mixture, a solution to be more precise, will retain at least qualitatively the properties of the two components, eg will be a disinfectant, a flammable liquid, a solvent, and so on.
Flammable materials are better stored in separated containers in separated places.
For the case in question, going camping with two small bottles or one double sized bottle in the backpack won't make any real difference.