Two 150 divine weapons with no (+) will not make a +1 weapon. You need at least a 150 divine plus a 150(+1) divine weapon to make that regular 150 into a +1 if you set it as base equipment in soul matching. To make a 150(+2) divine, you need at least two 150(+1) weapons. To make a +3 divine, you need at least two 150(+2) weapons and so forth. To keep bonuses, be max familiarity with the weapon you're trying to upgrade and set it as base material in soul matching. Sorry if my explanation is confusing, I'll try find video that explains better.
Edit: short random video explaining:
https://www.youtube.com/watch?v=p9fe2Vs3UyY