hi bob,

i would suggest that this can't be done hidden behind the scenes in a
conditional formatting formual. You will need to create another table
that 'checks' that two trainees aren't allocated to the same machine on
a given day. You can then use the value returned in this table to
conditionally format the results in your schedule table to highlight
problems as required.

i had a quick fiddle and got the following system working:

using the data you suggested above, except that i put the 'QUAL' field
next to the 'EMPL' field - it made more sense here, then you can create
new days just by adding a column and copying the formulas. with the
schedule table structured this way and occupying A1:D7, create a new
'check' table (either underneath or on another sheet - bear in mind
that you can only conditionally format using data from the same sheet)
like the following

Machine mon tue etc...
North ... ...
South ... ...
Central ... ...
etc...

then in the cells put the following formula (it is all one formula -
broken up for display purposes!):

=INDEX(($B$2:$B$7,C$2:C$7),
MATCH($B10,C$2:C$7,FALSE),1)
=INDEX((OFFSET($B$2,MATCH($B10,C$2:C$7,FALSE),0):$ B$7,
OFFSET(C$2,MATCH($B10,C$2:C$7,FALSE),0):C$7),
MATCH($B10,OFFSET(C$2,MATCH($B10,C$2:C$7,FALSE),0) :C$7,
FALSE),1)

The basic premise of the formula is to look up the qualification (colB)
of the first instance of the machine in the day-column (eg mon=colA)
and then check if this is equal to the qualification of the second
instance of the machine in the day-column. I've used the offset
function to only skip over the first instance and start from the cell
below it to find the second instance. It was slightly complicated by
making it 'copiable' so the range reference for the INDEX() function is
combined of colB and a day-column.

You can then use a relatively simple formula in the conditional
formatting to look up the relevant cell in the 'check' table, for
example:

=VLOOKUP(C2,$B$10:$D$12,COLUMN()-1,FALSE)

where C2 is the cell the formula is in, and B10:D12 is the 'check'
table.

hope that helps.

Rob