but please forgive me (others too) if I don't respond to your term definitions and other non technical messages anymore and it's not because I don't bother myself, because then we miss the core issue.
and
I'm not going to argue about meanings of terms because then we never come to a conclusion about the core issue.
NOBODY wants to argue what the correct term is, but for your posts to be understood it is important that the terms are correct to avoid misunderstandings, so PLEASE use the correct terms - at least after being made aware what they are.

ISR "demanding in which way?¡¨
Programming of it takes considerable memory, it is prone to error, it takes much processing time,...
That is not correct, if you use a processor with hardware IIC such as the 668.

Erik said "That would not help you much, one end must be a USB master and no '51 (yet) can do that.¨
I'm not sure I understood what is your mean, but from microcontroller in that specific sentence I didn't mean 8051..
Two USB slaves (all '51 implementations I know are slaves) can not talk to each other

Erik said
"¡§Using two FIFOs is hardly cheap¡¨ well in protocol that I've talked about I intended not to use supervisor chip so it may offset the price :)
That is bull... skipping the supervisor is ridiculous, you will get all kinds of problems. Besides a supervisor is ~$1 and FIFOs are much more.

Anyhow if a recommendation is what you want here it is:
Use chips with hardware IIC (P89C66x, P89LPC93x some Cygnals and more) The added cost of a hardware IIC is so much less than your FIFOs. Also the pin usage is two, not 11 and with 11 pins you typically run into all kinds of fun stuff if external memory is involved as well.

Erik

PS the FIFO solution is doable, but not cost efficient.