Having neither a tulpa nor being a drug user, theoretically I'm an awful person to answer this question, however, since you're unlikely to have a large user base who are drug users who also have tulpas, I figured I'd take a scientific approach to this question and answer it by looking at the effects of cannabis and therefore theorizing the effects that it could have on your communication.

Cannabis is primarily considered a hallucinogen but is also a sedating drug. Cannabis can make you more relaxed and happy and often makes people incredibly prone to laughter, it would only be natural to assume that this effect would carry on to your tulpa since you share the same brain space and the chemicals would affect both of you (though the question of the placebo effect could be called into play if you consider that if you think that the tulpa has a lower chance of getting high then you could actively create a situation in which you "block" (for want of a better word) your tulpa from getting high). This probably wouldn't affect communication between you and your tulpas though so let's examine the effects that would:

As I said Cannabis is a known sedative and part of the effects entailed in that is that it "can make it difficult for you to concentrate and learn, make your memory worse and make you feel less motivated." (http://www.talktofrank.com/drug/cannabis) Considering that the main method of contact with a tulpa (especially if they're a relatively young/inexperienced tulpa) is through concentration, this could serve to weaken your communication as the cannabis would impair your ability to focus properly in order to communicate. Depending on your view of tulpas (and where in the brain they exist if they are even gathered in a specific place (which I'm not a subscriber to but hey, let's be inclusive here)) the memory loss part could potentially have an affect upon your ability to communicate along with any other impairment of the brain. It can also cause anxiety and paranoia if used long term which could seriously damage your relationship (and therefore your communication with) your tulpa if you were being paranoid about it. If the affects affected both you and your tulpa then that could worsen the situation as both of you would be paranoid about the other 'betraying' you somehow. So there are the possible negatives to Cannabis use for a tulpamancer.

I also can't see many potential benefits, as I stated Cannabis is a hallucinogen which means that, by its very nature, it causes you to see, hear and feel things that are not real. Therefore, even if you thought that you had a good communication session with your tulpa whilst smoking Cannabis it could actually be that you were merely hallucinating the experience (and yes, there is a difference but I'm not going to go in depth into that) and that you weren't actually interacting with your tulpa at all.

In conclusion, I don't see many potential benefits to Cannabis use as a way to improve communication with your tulpa, however, having no first hand experience I could just as easily be proved wrong. The point of these threads is to spark conversation so I guess this could be an interesting way of doing that. As always, if I have made a mistake then I apologise and if you point it out to me I shall either defend my statement (if I feel that there is grounds for such an action) or I shall amend it.