I want to model a pair of boots for a character. I'll model one and mirror it across a YZ plane at X=0 to get two, but the boot itself is generally symmetric about an internal plane too (e.g. YZ through the middle toe). Is it possible to mirror an object across two planes at once? Thanks!
Yes, you can model the half of a boot and mirror it at the object's origin (orange dot) with a mirror modifier. Then just add another mirror modifier and specify another object in the modifier's settings that will be the origin for the 2nd mirroring. The object can be the character model or an Empty, for example.