That simple, implicit standard exists since RFC793:

  Simultaneous initiation is only slightly more complex, as is shown in
  figure 8.  Each TCP cycles from CLOSED to SYN-SENT to SYN-RECEIVED to
  ESTABLISHED.



      TCP A                                            TCP B

  1.  CLOSED                                           CLOSED

  2.  SYN-SENT     --> <SEQ=100><CTL=SYN>              ...

  3.  SYN-RECEIVED <-- <SEQ=300><CTL=SYN>              <-- SYN-SENT

  4.               ... <SEQ=100><CTL=SYN>              --> SYN-RECEIVED

  5.  SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ...

  6.  ESTABLISHED  <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED

  7.               ... <SEQ=101><ACK=301><CTL=ACK>     --> ESTABLISHED

                Simultaneous Connection Synchronization

                               Figure 8.

Huh, TIL, thank you!

Are you sure all firewalls support this? RFC 5382 seems to specify it, but then again, middleboxes aren't exactly known for strict RFC compliance...