In Hans Oerberg's book Familia Romana you can find this sentence: īnfrā pulmōnēs est iecur et venter (Ch. XI; lines 20-21) - shouldn't it say "sunt iecur et venter". We have two subjects in the sentence so why is there the verb in singular?
Because there isn't really a rule in Latin that says that the verb must necessarily be plural if there are multiple subjects (even if Allen & Greenough give precisely such a rule).
Now that is generally true if the subjects are all persons, so you have your Marcus et Cornelia mortui sunt (but: neque Marcus neque Cornelia mortua est). But if not, then the predicate verb usually agrees with the closest subject:
In periculo castra et legiones et imperator erat. (Caes. BG 2,26,5)
Populi provinciaeque liberatae sunt. (Cic. Phil. 5,12)
There are many exceptions. In particular, multiple thing-subjects of different genders are sometimes seen with a neutral plural predicate, so my understanding is that infra pulmones sunt iecur et venter would also have been correct.