I have mixed different Lots of Varget on a few occasions. Like Drew (skiutah02), I believe it has to be mixed uniformly for best results. Also similar to Drew, I am a biochemist, so mixing/blending materials is not an unfamiliar process. I have been loading from a batch of about 20 lbs of Varget "Mix" for the last couple years, and it works just as well in my hands as a single Lot.
I purchased a plastic 5 gallon paint bucket at Lowes with a lid that has a rubber o-ring seal and closes tightly. I basically "layered" each of four different Lots of powder successively in the bucket, one pound at a time, in repeating order, one on top of the other until all the powder was in the bucket. In other words, the mixing process actually started long before I began shaking the bucket. After all the powder had been layered into it, the bucket was perhaps 2/3 full. It's VERY important to leave plenty of extra room in the bucket for the powder to move around so it can mix uniformly. I snapped the plastic lid with the rubber o-ring into place, then taped it off with packing tape so there was no way it was coming open. I began the mixing process by mixing/inverting the bucket end over end. Then I would roll it a few turns. Basically, I left the bucket sitting on the floor in the middle of my living room for a couple days, and would do the end over and rolling thing for a few shakes every time I walked past it. It was quite well mixed by the time I was done. I also took samples from various regions of the bucket and compared them for velocity in loaded rounds, which turned out to be quite uniform. Using a large automotive plastic oil funnel, I dispense the Varget from the bucket into an old 8 lb jug as needed.
It is possible that like many things I do with respect to reloading, this approach might be a tiny bit, shall I say, "excessive". Nonetheless, it works. I should also point out that I did not vigorously or roughly shake the mixing bucket. A gentle tumbling or rolling is all it takes, and if you're going to mix the powder periodically over a couple days like I did, shaking the bucket too vigorously can actually damage the kernels. So there's no reason to overdo it.
I should also add that prior to mixing this particular batch of Varget, I had determined velocity for each individual Lot in side-by-side testing, so I had a pretty good idea that all of the different Lots were generating velocities for a given charge weight that did not differ by more than 15-20 fps total. In other words, they were fairly close Lots to start with. Mixing together two Lots individual that represented High/Low extremes with regard to burn rate and velocity might be a little more problematic, but with uniform mixing can still be done.
Regardless of whether you're dumping the last little remainder of Lot into the new Lot of powder, or purposely mixing together pound quantities to create a single large batch, uniform mixing is the key to making sure the resulting mixture behaves as a single homogenous and consistent batch of powder.