Cannabis mixes well, but also has the potential to make the trip more volatile. That is to say, it may aggravate confused or manic thought patterns, create undesirable feedback loops, intense self-awareness, etc. It doesn't have the same qualitative synergy as cannabis + serotonergic psychedelics for me, it's not quite as essential, but it'll definitely take you further out for better or for worse.
For a first try of this combo, I wouldn't hit any weed while you're still re-dosing and/or coming up on the k -- wait until you're as high as you're going to be on the ketamine. If you're comfortable with that level of high, having a good time, comfortable in your environment, etc, then it'll most likely be groovy. If any of these variables is off, I'd put the peace pipe down until you're coming down, bearing in mind that smoking on the comedown also has the potential revive peak effects of the ketamine. Proceed with caution and lightness, if things go askew breath deep and drink water.