Is Assumed Temperature used in conjunction with a Derate in the B737 NG?
Yes, that is allowed.
You can combine selection of a fixed derate and an assumed temperature for the takeoff thrust on the FMC N1 Limit page, but only up to a reduction of 25%.

Takeoff Derate
Fixed derates can be selected on the N1 LIMIT page. [...]
Derated takeoff rating can be further reduced by assumed temperature. [...]
Assumed Temperature Thrust Reduction Takeoff
A takeoff thrust less than the full rated thrust may be achieved by using an
assumed temperature that is higher than the actual temperature. The desired thrust
level is obtained through entry of a SEL TEMP value on the N1 LIMIT page or
TAKEOFF REF page 2. [...]
The maximum thrust reduction authorized is 25 percent below any certified rating. [...]
If the assumed temperature method is applied to a fixed derate, application of
additional power should not exceed the fixed derate N1 limit as loss of directional
control could occur while on the ground.
When the assumed temperature method is used with full rate, the reduced thrust
setting is not considered a limitation. If conditions are encountered where
additional thrust is desired, the crew can manually apply full thrust.

(Boeing 737NG FCOMv2 11.32.3 - Flight Management, Navigation - Flight Management Computer)
You can see the current selection on the Upper Display Unit:


2 Thrust Mode Display
[...] Thrust mode display annunciations are:

TO – takeoff
TO 1 – derated takeoff one
TO 2 – derated takeoff two
D-TO – assumed temperature reduced thrust takeoff
D-TO 1 – derate one and assumed temperature reduced thrust takeoff
D-TO 2 – derate two and assumed temperature reduced thrust takeoff


(Boeing 737NG FCOMv2 7.10.2 - Engines, APU - Side by Side – Displays)
What would actually be used by an airline depends on the exact situation. The derate and the thrust reduction via assumed temperature are not the same thing because the derate affects Vmcg, but the assumed temperature does not (see also What is the difference between Assumed Temperature and Derate takeoff thrust?). Today, most airlines will use an EFB (or a separate tablet computer) to perform the TO calculations and the computer will optimize the thrust reduction while taking all legal requirements for V1 (which may be affected by Vmcg, which is a lower limit for V1), second/fourth segment climb gradient, obstacle clearance and possibly noise reduction into account.