Do you have previous experience with either of them? It is always smart to try something on its own before combining, and especially in the case of Amanita Muscaria I would almost call that mandatory.
Have you read about Amanita's? They are weird ones, not typical psychedelics really... they need to be prepared properly, there can be a miriad of side-effects and potency is unpredictable unless you make a standardized product out of it.
What would be the point to combine them? Is it because you don't have that much San Pedro? I wouldn't take them together, or if I did I would need to have extensive experience so that I would know what to expect from the separate substances, and how to handle the unpredictable potency.
20 g of dry San Pedro at say 0.5% potency would give you 100 mg of alkaloid, is that right? It would be very mild. Also a tea is not easily prepared from cactus powder IME, turns into slimy sludge.
I think you have a road ahead of you of doing research on them separately, be careful with the Amanita's and try to get more cactus to have a good dose, or just try the 20g as a mild introduction to the feeling. Mescaline and co are pretty gentle on the mind IMO so I think slow titration is less imperative than with Amanita's.
good luck