How do I get both parties to access each other's files at the same time?

Question

Encryption

Good day to all!
Faced with a non-trivial problem.
Company. in which I work, when communicating with some contractors, it exchanges pdf files. At the same time, it is necessary for both parties to be able to see the data in each other's files at the same time (a matter of trust, there are fears that if one side sees the other's file earlier, it can change its file, as it will be beneficial for it).
At the moment, they send files to each other at the same time, to the "One-two-three" account.
There are the following restrictions:
1) It must be an independent resource, or a program (if we write the program ourselves, the other party will not be sure that we did not leave a loophole for ourselves)
2) This resource / program should be easy to use (people who are far from IT send letters, + it is necessary that counterparties agree to this method).
What can you suggest? Thanks in advance!

Answer 1

There are so-called Special Operators of Communication.
They do pretty much the same service.
Look, perhaps one of them has just such a service.
PS Re-read your own answer again. If privacy is not important, then what you are looking for is a trusted time server (TSA). Each of the senders sends him a hash sum from the document at any time before 12:00, for example, at 11:15 and 11:35, and receives his signature over the block [hash sum; time imprint on his watch]. And then after 12:00, without much concern for synchronization, the parties exchange documents and signed hash amounts with time stamps. This ensures that no changes are made after 12:00.
certsrv.ru/cryptoarm_user_manual.ru/topic_1001.htm

Answer 2

send "much in advance" a file in a rar archive, 8-12 characters, they won't peep in a day