The method of blending two megascans materials has changed slightly in Unreal Engine 5. Instead of finding the blend in the old style megascans menu, it has now moved into the bridge app itself. You can use the methods listed here to blend materials and add puddles to this blend easily with a few click and some vertex painting.
Steps
Select both your material instances in the content browser that you have already imported into Unreal
Open Quixel Bridge from the add content drop down menu
Click the settings button next to the quality setting/add button
Click on "Create material blend" under material blend settings
(Optional: Enabling water puddle in created material)
New material will save in 'BlendMaterials' in root folder
If necessary, increase poly count of your desired puddle mesh with "remesh" in modeling panel
Go into mesh paint mode (Shift+4) then paint using red channel for secondary material and blue channel for water
Screenshot Guide
