Direct answer
WheelMates is an exactly two-player game; it does not have a solo mode. Firevolt’s pre-launch Q&A explicitly said the studio had no current plan for one-player support or groups larger than two.
Each person controls a separate miniature RC car. Puzzles are built around communication, timing, and both cars performing distinct parts of a solution, which is why the second player is a requirement rather than an optional helper controlled by AI.
Ways to supply player two
Two owners can play online, two people can use the listed shared/split-screen co-op mode, or one owner can invite a partner who has downloaded the free Friend’s Pass. The game does not include public solo progression as a substitute for those options.
Avoid a misleading store assumption
Steam does not list a separate “Single-player” feature for WheelMates. Community pages written before Firevolt’s Q&A sometimes infer solo support from free exploration; that is incorrect. Free exploration means the pair can revisit and cruise through unlocked locations without following a puzzle objective, not that one person can run the campaign alone.