Hello,
Return code 748 -> access to registered server denied
I'll try my best to clarify 
.
You have an external system that is registering the program "MYPROGRAM" at the gateway from the SAP host A.
Thus, the only reginfo file relevant here is the one from host A. The reginfo from host B plays no role.
Now, if you only have the following lines at the reginfo from host A:
P TP=* HOST=* ACCESS=local
P TP=* HOST=* ACCESS=internal
The host B will not be able to communicate with "MYPROGRAM".
That is because the gateway will use the first rule that matches to accept the registration and the first rule has "ACCESS=local", meaning that only host A itself can access this program.
Now, if you change the order of the rules to
P TP=* HOST=* ACCESS=internal
P TP=* HOST=* ACCESS=local
It will work from host B as well (after the de-registration / re-registration).
An even better option would be to create the reginfo with entries like (notice that the second rule is too large and SCN breaks the line):
#VERSION=2
P TP=MYPROGRAM HOST=<server that registers this program> ACCESS=internal
CANCEL=internal,<same as in HOST>
P TP=* HOST=local,internal ACCESS=local,internal CANCEL=local,internal
A different scenario could be that this external system is performing the registration of MYPGORAM at every SAP instance (meaning on both host A and host B).
In this case, both reginfo files (from both hosts) would be relevant.
Regards,
Isaías