Avalon bus with 2 Avalon_MM_ Slaves fails to work (works fine with 1...)

I would avoid using chipselect for starters. This is from the spec, pay extra attention to the last sentence:

"When present, a slave port ignores all Avalon-MM signals unless

chipselect is asserted. chipselect must be used in

combination with read or write. The chipselect signal is not

necessary; Altera does not recommend using it."

Have you simulated your system to see what is happening at the fabric level? That's typically where I start when debugging issues that resemble this.