According to Hutchings (from his book) 250ml will develop a roll of 35mm or a sheet of 8x10. Therefore, you could safely develop your two rolls in a 500ml tank.(posted 8223 days ago)I'm a bit conservative, so I usually develop 2-3 rolls in a 4 reel (one liter) tank. Using PMK I always fill the tank. If you fail to do so, the developer tends to oxidize too quickly and you will have difficulty getting consistent results.
As for fixing, enough fixer to cover the reels comfortably is enough. On the other hand, why bother trying to skimp? Use enough to fill the tank; you'll be emptying it back in the fixer jug anyway.