I have personally found that Salvia and weed work GREAT together and I think (though I could be wrong) that there is a legit synergy between the 2, or at least if not chemically I found that they complement eachother.
HOWEVER...If you are going to do it you don't mix them in a joint.
The only good way to smoke Salvia is to take MASSIVE bong hits of them and hold them in as long as possible.
I don't know why, but it's the only real way to make it work so putting it in a joint is useless.
What I recommend and which I tried, was to first smoke a few bowls of weed and then take a few big bong hits of Salvia;
Or, you can first smoke a bowl of weed, then put weed with salvia in a bong and take big hits, which works well.
If you are set on smoking joints of weed, fine, then smoke a joint or two first, then take bongloads of Salvia, but putting Salvia in a joint is probably not going to do much.
I have also found that the weed + Salvia + Nitrous mixture, or even just Salvia + nitrous is INCREDIBLE.
And FYI, Salvia gives the best visuals and auditory hallucinations in complete or at least partial darkness and silence.
This is how it was used as a divinatory agent called "Ska Maria Pastora" by indian tribes in Mexico and it's how I usually did it.
If you have the lights on high you may not even get visuals from Salvia and instead just feel fucked up or get auditories, or at least that is what I found.
Edit: AND YES....Salvia only works for about 10 mins...maybe 15-20 MAX or even 5 mins minimum....
If you want a longer experience you will have to smoke more and weed won't make the experience last any longer though I have heard that sublingual extracts make the salvia experience last longer but are milder, though I have never tried them.