"We stand around the tall tree, on which a bear and a tiger are sitting."
Translation:A körül a magas fa körül állunk, amin egy medve és egy tigris ül.
This seems to be the Hungarian way :) The two subjects are each singular, so the verb stays singular.
I think both should be accepted, but ül is a lot more natural. Your subjects, especially the one closest to the verb, are singular.
EDIT: scratch that, stick to the singular unless you have multiple proper nouns as subjects, then anything goes.
There is such a thing as grammatical (syntax) matching and matching by meaning (semantics?). It is mainly a matter of preference. If all subjects are singular, it is good to use a singular verb, but some people prefer the plural. If there is a plural subject, the verb is more likely to be plural.
The rules are flexible here.