Do bonuses stack if they come from 2 identical magic items that don't require attunement? In particular using 2 transmuter stones with 2 different effects.What if both stones give 10 speed?
No, the bonuses will not stack. Though it is not present in original printings, errata to the Dungeon Master's Guide (and later reprintings of the book) add a section on page 252, Combining Game Effects:
Different game features can affect a target at the same time. But when two or more game features have the same name, only the effects of one of them—the most potent one apply while the durations of the effects overlap. For example, if a target is ignited by a fire elemental’s Fire Form trait, the ongoing fire damage doesn’t increase if the burning target is subjected to that trait again. Game features include spells, class features, feats, racial traits, monster abilities, and magic items. See the related rule in the “Combining Magical Effects” section of chapter 10 in the Player’s Handbook.