pathfinder 1e – Do multiple baldric, bane items stack?

No, 2 bane baldrics don’t give +10 to inquisitor level

For one thing, a baldric is a chest-slot item. You can’t wear two at the same time. Whether or not you could combine two into one item, or create a bane non-baldric, is entirely up to the GM, and per the guidelines would cost 50% extra.

Even assuming your GM does let you get around that (and in the general case, combining or moving item effects should be a relatively low-friction ask of the GM—even the 50% extra that Paizo recommends is far, far too much in most cases), it wouldn’t help because both bane baldric effects are the same source. Just like having resist energy cast on you twice doesn’t give you resistance 20, having two bane baldric effects aren’t going to stack.

Technically, strict-RAW would have some pause here simply because there are rules about (non-)stacking bonuses, and (non-)stacking spells, but the more general case of non-bonus benefits from things other than spells isn’t directly discussed. However, the absence of rules on the subject (either way) doesn’t inherently allow anything; it just means you need a ruling from the GM. And the only plausible ruling I can see a GM giving is one that’s consistent with all the things we do have rules for. After all, for all that rule says “spells,” it’s in the “Magic Overview” section. Anyway, that rule is

Same Effect More than Once in Different Strengths

In cases when two or more identical spells are operating in the same area or on the same target, but at different strengths, only the one with the highest strength applies.

In this case, they don’t have different strengths—they have exactly the same strength—but again, it’s implausible to expect any ruling that acknowledges this as a legitimate “loophole” in the rules and allows these to stacks.