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This article should help you:
http://hort.ufl.edu/database/documents/pdf/tree_fact_sheets/senspea.pdf

That is really up to you. It is more of an aesthetic choice than one for the health of the tree. You can remove one, if you don't want 2 trunks, but the tree will be fine too if you leave them both.

This article will help with caring for your tree: https://www.gardeningknowhow.com/ornamental/trees/cassia/growing-cassia-trees.htm

Neem oil is both an organic insecticide and fungicide should either issue be affecting your cassia. Here is more information: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

Hopefully, this article will help; https://www.gardeningknowhow.com/ornamental/trees/cassia/growing-cassia-trees.htm

It sounds like it has a fungus. Treat the plant with a fungicide, like neem oil, and this should help to clear it up.

"These articles should help determine if the tree is sill alive: https://www.gardeningknowhow.com/plant-problems/environmental/how-to-tell-if-a-plant-is-dead-and-how-to-recover-an-almost-dead-plant.htm,
https://www.gardeningknowhow.com/ornamental/trees/tgen/what-does-a-dying-tree-look-like-signs-that-a-tree-is-dying.htm"