You CAN train the same muscle group two days in a row. In fact, that's best for optimal gains.
This method of double stimulation prolongs muscle protein synthesis, leads to enhanced feedback in muscle, and triggers greater muscle responsiveness. The second session is there to enhance the anabolic response from the first session. It does this by prolonging the period of increased protein synthesis and also increasing nutrient transport to the muscles.
How to Do It
Train the same muscle group two days in a row, but not in the same way. Hit it hard with heavy lifting the first day and then do pump work for the same muscles the next day in the first 15 minutes of the workout. Then you move on to your main workout for another muscle group. Here's an example using a 4-on/1-off split:
Day 1: Train the pressing muscles (think bench press) hard and heavy.
Day 2: In the first 15 minutes of the workout, hit the pressing muscles again with lighter pump work. Isolation exercises done in circuit fashion work best. Think constant tension for 8-12 reps. Each set should last at least 30 seconds and up to 50 seconds. Then do your main heavy workout for the pulling muscles (back work.)
Day 3: Pump work for the pulling muscles you trained hard the day before. Then train legs.
Day 4: Pump work for the legs. Abs, conditioning (optional)
Day 5: Off
This actually facilitates recovery and lengthens the duration of the anabolic phase. Protein synthesis stays elevated for 24 hours post-training, but by having a second session the next day that's less traumatic, you extend protein synthesis significantly, thus building more muscle.
Remember, it's important that the second session is pump work and not heavy lifting. We don't want to cause any muscle damage on that second day. We only want to activate the cell signaling responsible for stimulating hypertrophy and pumping nutrients into the muscles. For best results, use proper workout nutrition.
For greater detail and more training options, check out the related links below.