from my decades of patching, Ive only ever put one layer of glue on , and never an issue.
as said, proper sanding, enough glue to cover patch size, let dry, roll on patch to eliminate air or bumps, hold firm with thumb for a few minutes, and bingo.
I too used to apply 1 layer, but once had a hard time patching a stubborn tube and fixed it with 2 layers. Been patching that way since then, it's not much more work.