Just curious whether you mean mixing the two oils together in a medium or painting with walnut oil as a medium on top of passages that were done with linseed oil?
Also are you using straight walnut oil as your medium, or is it mixed with something else?
My initial reaction is that you should be fine since Walnut Oil is fatter (slower drying) than Linseed, but it might not be a good idea. I have read that it is better to use the same medium through the entire painting. That being said, I have used paint with walnut alkyd medium
over linseed oil only layers with zero problems.
I am assuming that you know that straight walnut oil mixed with the slower drying colors can take many weeks to just skim over, let alone actually dry. You may want to add a bit of walnut alkyd to the pure walnut oil, or even a bit of linseed oil to speed up the drying.
You might want to post this in the oils / technical area also to get a larger viewing audience.