I found a bit more in "An Introduction to Japanese Linguistics" by Natsuko Tsujimura, which notes that in the case of verbs with two が 's, the first has to be the subject & the second the object. It refers to the types of verbs that take two が as
stative verbs. Obviously 'aru' and 'iru' are examples.
There's also some useful bits in one of the student exercises in semantics. The second of these uses a na adjective (うまい). I'll quote some of that (maybe I'll learn it too that way):
Japanese is famous for having what are called multiple subject sentences. Consider (a-b).
a) Tanaka-sensei ga musuko-san ga nakunatta.
(It is Professor Tanaka whose son has died)
b) Yahari natsu ga biiru ga umai.
(After all, it's during the summer that beer tastes good)
In (a-b), there are two NPs that are both marked with the particle that typically goes with the subject. It has been claimed that the second NP in each of these sentences is the true subject, whereas the first NP-ga is to be distiguished as a MAJOR subject. The first NP-ga typically receives a stress peak and is followed by a pause
It goes on to give examples that are semantically anomalous.
The only problem with this book is that it is written for linguistics people and keeps using explanations and long words that the grammatically retarded yours truely struggles with.