Is it gross to wear the same outfit two days in a row?
No - unless you sweated in it or it touches skin directly. Airing overnight resets most pieces.
The hygiene question is about skin contact and sweat, not the calendar. Outer layers - blazers, cardigans, trousers, dresses worn over a base layer - barely touch skin and are fine for consecutive days, better still if aired on a hanger overnight. Base layers (tees, shirts worn against skin, underwear, socks) earn a wash after a full day. Socially, consecutive-day repeats are the least-noticed kind because people's memory for outfits fades within hours, not days. The exception that actually matters: summer heat or a commute that leaves you sweating - then everything that got damp gets washed.