Can one co-executor be forced to step back from a decision where they have a personal conflict of interest?

Yes, and it's really an extension of a trustee's basic fiduciary duty rather than something new. Where one co-executor has a personal stake in a specific decision — for example, they want to buy an estate asset themselves, they're a creditor of the estate, or a close family relationship affects their judgment on one particular issue — they're expected to disclose that conflict and generally should step back from making that specific decision.

In practice, the other co-executor, or executors, can usually proceed on that particular item without the conflicted one, provided the will's structure allows decisions to be made without total unanimity in that circumstance, or the conflicted executor's voluntary recusal effectively resolves it. Where the conflicted executor won't step back voluntarily, or where all the named executors share the same conflict, beneficiaries or the other trustee can apply to the court for directions on how the specific decision should be handled, potentially including appointing someone independent for that issue alone.

Because conflicts of interest in estate administration can be subtle rather than obvious, it's worth raising any concern about a co-executor's personal stake early and directly, rather than waiting until a decision has already been made.