What does it mean when the mcv is high and b12 is also high? what could this be?

Many things: I wouldn't worry so much about the high B12 level, as that isn't so much a cause of the a high MCV, in fact the opposite is true(low B12 levels cause high MCV). But a high MCV, means your red blood cells are large. This can be related to many things, from a higher number of immature red blood cells, underactive thyroid, too much alcohol, liver disease, low folic acid just to name a few.