RTL Viewer

architecture is selected when you instantiate the entity. If you do it via a component you need to use a configuration to select the appropriate architecture

component my_comp is ....
for all: my_comp
  use entity work.my_ent(arch_sel);

Or better, use direct instantiation:

inst1 : entity work.my_ent(arch_sel);

If you have two architectures at the top level, apart from asking why, I dont actually know how Quartus behaves. Multiple architures, when used, are only usually used for differentiating a simulation model of an entity verses a RTL implementation. I have never used multiple architectures and it is highly discouraged in industry. If you want different behaviours then create different entities (or use generics to control behaviour).