If both products have the exact same amount of pixels (colums and rows), you can interchange them in the band maths (described here)
But in your case, the producs are probably of different origin and resolution (optical and SAR) So your only option is to bring them into a stack of images. Your radar images must be geocoded to the same coordinate reference system as the optical images (NDVI) and then you can use either “create stack” in the coregistration menu or the “collocation” module. The differences between both products are explained here: Co-registration of Various Products from Different Satellites
The whole topic was also discussed here: Merging of two different satellite data