check the datasheet, if the ES1 bit is IE.6, you are good.
Yes, ES1 is IE.6

scan your TOTAL code for IP, EA, ES1 just to see if there is some overlooked change
No mention of IP at all.
No mention of EA except the one setb EA instruction in the reset initialisation.
ES1 is set in the reset intialisation, cleared at the beginning of the interrupt handler and set again just before the RETI. No other mentions.

NO, absolutely NOT, it means that if RI0/TI0 and RI1/TI1 are active at the same time UART0 get priority. UART0 can not interrupt a UART1 ISR unless IP is set.
I see what you mean. Thanks for clarifying this. The wording in the Dallas User Guide is I think misleading:
The word "priority" seems to mean two different things:

1. What can interrupt what, and
2. Which wins if two interrupts occur at the same time

However, I don't think any of this explains why my serial port 1 is not interrupting.

Thanks - Rowan