When you do an XC START TRAN(Y) TERM(x) , the 3270 terminal is open for keyboard-related activity. Thus, if the user gets in quickly, they can initiate transaction Z before Y gets going. This is because, in VTAM terms, an End Bracket has flowed from CICS so permitting keyboard input.
In your case, it looks like the transaction initated by PF2 has got going before the one previously started by PF1 so causing the failure.
You get around this by not doing an XC START but an XC RETURN TRANSID(B) IMMEDIATE. This will start the B transaction at the terminal without ending the Bracket. The terminal will not be unlocked, and so the use of PF2 will not start its transaction as there is not an opportunity for the user to interrupt processing.
Transaction B had better check for PF2 after its first interaction with the user to reject usage.
This was first published in October 2002