Hi Rachel,
There are a few things to consider here. The bands between Landsat 5 and 8 are for sure different. For L5, bands 4 and 3 are needed for NIR and Red, while for L8 it is bands 5 and 4. Also, ideally, each image you are using to produce the NDVI product should be corrected to a surface reflectance product otherwise there will be lots of variability in the result that is false.
You asked about a model, how do you mean? If you have a mosaic dataset for L5 data, and a mosaic dataset for L8 data, you can construct raster functions to produce the NDVI product for each of those, and then difference those NDVI products. That can be done either in python or model builder, I believe.