I'm not exactly sure why you are having a problem...
You use an AXI BRAM Controller on BRAM_PORTA, as you show in your first post, and make the BRAM_PORTB an external if you want it to connect to something outside the BD. Otherwise you if your other code is going to be a new BD IP then you don't need to make it external just connect it to the custom IP.
e.g.
View attachment 136984
As you can see in the picture the Block Memory Generator IP has the native BRAM interface signals, which can be used in the BD or outside the BD like I've shown. Or you can do it like you did with separate signals, either way will work.