Of course, "practicing" is a loose term (as is "rivals" -- more like "cannon fodder"). What the Chiefs and Chargers can really do is begin in-house offseason conditioning. Quarterbacks can throw to uncovered wide receivers, and strength and conditioning coaches can help guys stretch to their very best, but other than that, it isn't all that interesting. Coaches aren't even present, and footballs can't even be used. They're the same Phase I restrictions that will be imposed upon the Denver Broncos once their workout program begins April 15.
(The difference between these workouts? The Broncos' will involve Peyton Manning throwing to Wes Welker and Jacob Tamme. That's a bit more interesting than Philip Rivers having another comeback year in April or Alex Smith getting his playbook.)
In the end a good turnout is expected (of players -- these are close to the public), and we're one step closer to football. Even if that step is just a closely monitored lunge stretch.