Put some thermal paste on both sides of the mica washer.
I have not checked on-line, but it looks like both your devices have their heatsinks fully encapsulated. If this is true, you don't need to use the mica washers at all. Just place some thermal paste between your devices & the heatsink.
To be safe, check the insulation specs of each device to ensure the device voltage insulation is satisfactory for your application. You might need to use a plastic sleeve for the screw & the nut. As always, check the manufacturer's datasheets.