Here is an approach we used, using serial comms:

A PC is running a scope/logic probe application.
A PIC manages a single 8 bit scope probe.
A PIC manages 8 logic probes
The PC can manage several PICS (8 max in our setup), all connected to the
same serial line.  All PICs are also joined by a single 'trigger' line.
Any PIC can be a trigger (scope value above/below/rise/fall,
Each PIC has an address.
All PICs keep the serial TX line input unless they are sending data.
We have a command structure with an address byte
We send setup to one/all PICs: address FF = all for common stuff, address
in range 00-07 to talk to one PIC.
A command is sent to one PIC to tell it the trigger condition.  Only it is
a trigger device,  it sets its trigger line output low.  The other PICs
switch their trigger line input.
All PICs collect samples.  When the trigger PIC hits its trigger condition,
it raises its trigger line.  All PICs collect the required number of
samples (as per the command).
When the trigger PIC has got all the samples it needs, it sends a
'complete' message to the PC.
The PC sends a 'return data' command to device 0.  It sets TX output and
sends its samples, then sets the TX line input again.  The PC repeats for
all devices and displays the data.
We use similar protocols to 'query' devices on start up:
PC sends 'query device' to device 00.  The PIC returns its type (scope,
logic, etc).  If no reply within 0.1 seconds, the PC knows there is no
device with that address.
I'm leaving out a few things, like pull downs and so on, but I'm sure you
can see what we're doing from this description.
What we have is basically a multi-dropped line, with a software protocol
controlled by the PC as to which remote unit on the line can talk.  I'm
sure there are other ways of doing this, but what we have works!!!
We have other multi-PIC setups: some using radio loops to link PICs (our
basic product range).  Here is our most complex setup, which we used with
smaller PICs, for a radio loop with a remote keypad:

Radio PIC---serial link---LDC/Keypad PIC---Serial link---Memory mangement
PIC (menu text)
Radio PIC---serial link---Management PIC---serial link---memory management
PIC (system control memory)
Several Radio PICs in a loop carrying out a security application

This is a thing of the past, we've simplified our system a lot, mostly by
using larger PICs, but we managed to make quite a flexible system out of a
lot of PICs, joined by serial and radio links.

Ed Todd <edtodd@sni.net>