If I use 90% of the allowed maximal dosage of a local anaesthetic "A"
(such as Lidocaine) on a patient, how much of a second, different
local anaesthetic "B" (such as Bupivacaine) can I use? (10% or more of
the maximal allowed dosage?) Where can I look this up?

Thank you very much for your help.

Generally local anesthetic toxicities are strictly additive. I.e.,
using one agent to 90% of its toxic limit allows another agent to be
used to only 10% of its toxic limit. This is knowledge acquired some
years ago; sorry I cannot produce any references just now.