Christian:
There are microcontrollers available that have BOTH SPI and I²C on board. These same parts can be had with sizeable onboard FLASH and XRAM as well. It seems to me that when you elect to use the off-board PCA9564 part that you are probably using external BUS mode. This sucks up lots of I/O pins and board space. I'll bet that if you look at cost (including total product cycle costs) that the single microcontroller solution will be a huge advantage.

It is not uncommon for some people to try to use the 8051 or one of its derivatives more like a micro-processor with a full external bus structure. This is the scheme promoted in many 8051 books wherein the old fashioned low cost debug technique using a ROM Monitor over the serial port and having the code in RAM at 0x8000 was used. However in todays world some of the very same microcontrollers I mention above support a much more user friendly type of debugging right from FLASH. If you take advantage of this scheme your development is much more enjoyable and the resulting product is much more cost effective. Look here and here for some ideas about using these more modern parts.

Michael Karas