BOTH machines exhibit this exact same behavior after swapping?
Not even a slight, momentary, attempted spin-up of any fan?
(Door open to view this.)
Are you depressing the power button for more than just an instant?
Battery still with acceptable range voltage present?
CPUs properly seated?
NOW, what exact Model numbers are these two machines?
Like… M8570, M9145 OR M9309... and with number / type of corresponding FW ports?
(i.e. FW400 and / or FW800) for each model number.
AND… what Apple number is on your CPU daughter cards?
Pictured here is a 1 GHz SP CPU from a FW400 MDD.
(Number within green rectangle.)
I know, a PITA to remove heatsinks and repaste.
But, if necessary…
(Don’t seem to have a 1 GHz DP CPU upon my first search here.)