Hi,
Does anyone know if I can stack a two tier square cake without using dowel rods? Would it depend on cake firmness or the type of cake?
Thanks

No. Almost any cake stacked on top of another should have support. There are several options as to what is used - plastic drink straws; hollow 'pillars'; wooden dowel rods and others. The cake that is going on top needs to be on a cake board or it would just sink into the support
The *only!* time one does not need support is if it is a *very small* cake - like a cupcake or 3 or 4"er.
For 6"x3" rounds (which were mostly what I used) I only used 3 plastic drink straws as support.

Single-layer tiers wouldn't require supports... everything 4" and over in height does.

And to support you need the dowel rods/straws/whatever AND a cardboard. One does no good without the other.