In the chapter on Spellcasting, there is a section called "Combining Magical Effects". However, despite the seeming generality of the title, the text of this section only talks about magical effects from spells:
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect--such as the highest bonus--from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap.
It makes sense that this section would only talk about spells, since it is in the chapter about spellcasting. However, this seems to leave ambiguous the question of how magical effects from sources other than spells combine. Such an issue could arise, for example, if two different items (or two of the same item) grant a bonus to attack rolls, or a bonus to the same saving throw.
Is there a published rule or official ruling that governs how non-spell magical effects combine?
(This question was inspired by another question about the interaction between +X magical ammunition and Oil of Sharpness, and could be considered a generalization of that question.)