Within 1 minute counts as simultaneous
BUT
I found instructions for Revell 03014 (almost the same bits and pieces)
here:
https://www.scalemates.com/kits/132626-revell-03014-105mm-feldhaubitze-l scroll down a bit and there is a greenish "button" to download a pdf-file, I have tried and it works.
The Revell boxing seems to have been upgraded with a muzzle brake and modern rear lights. The instructions should work together with the plastic in the Testors boxing.
Step 12 says that the fron part of the barrel should be cut off. This applies to the modernised German version (muzzle brake) so ignore this and keep the original parts.
If I read the instructions correctly the parts with numbers above 88 shall be ignored for the Testors boxing
/ Robin