It can be complicated.
Operations can be scheduled or unscheduled, and flight crews can be scheduled or unscheduled. One may be part of a regularly scheduled crew in an unscheduled operation, or one may be an unscheduled crewmember in an unscheduled operation.
The original poster asked about duty times for "2 crew unsched under 135," which could encompass either or both unscheduled crews, and unscheduled operations.
In the case of unscheduled 135 operations using a crew that does not have a regular duty schedule, there is no duty time. The only restriction is rest time, which will always be a minimum of 10 consecutive hours.
The ability to exceed flight time limits granted under 135.267(c) is a function of a crew with a regularly scheduled duty. It's not applicable to duty limitations, but only to flight time limits.
A crew without a regularly scheduled duty doesn't have a duty time limitation. The crew operates under the same rest restrictions that a single pilot operation has, however...10 hours in the previous 24 hours. Accordingly, one may only have 14 hours of duty without exceeding the rest limitations.
One may exceed flight time limits, but not duty limits...and as there are no duty limits spelled out for a crew without a schedule, one can't exceed rest limits. One must be able to look back 24 hours at any given time, and find 10 consecutive hours of rest (free from all duty to act for the employer) during that time...this requirement applies constantly at any given time during the day...at 3:00 in the afternoon one must be able to look back 24 hours and find those 10 hours of rest. At 5:00 PM one must be able to look back 24 hours and find those 10 hours of rest, etc. The rest requirement (and limitation) is the guiding principle; duty limits aren't prescribed...and exist simply as a function of of the remaining hours of the 24-hour look-back period.