It depends on the CM at the FP machines.
To the best of my knowledge, the "not a valid fp" message that gets printed on the tickets does not indicate why it's invalid; unless the reason is that "You currently hold an active FASTPASS ticket for.... Another FASTPASS ticket will be available after...." In other words, it doesn't say that it's invalid because the tickets were not used for admission earlier that day (which might be a good idea for Disney to implement in order to assist CMs). "De-magnetized" tickets also result in "not a valid fP" error message, even if they were used earlier in the day for a valid guest entry.
In my experience, when one ticket works but the other does not, and the reason is not due to holding an active FP (and not waiting the allocated time period before retrieving another), the CM will kindly issue a valid FP for the non-functioning ticket, along with a recommendation that the ticket probably needs to be replaced due to demagnetization or the like.
It's common for a "representative" of the family to go to the FP machines with all passes in-hand, instead of everyone in the group lining up to get their passes one at a time.
But if you have a handful of tickets, and only ONE of them is functioning (because you used it to enter the park), and the rest are all showing as invalid, and it's 9:20 AM, the CM may politely inquire as to whether the other individuals are indeed in the park, and at that point, may request that they be present in order to override the FP machine.