I just did an 9 A.M. crit followed by a 2:00 p.m. CX race. I didn't feel terribly drained during the 2nd race but my mind wasn't there. I kept thinking "how would I feel if I didn't race that first race". I had a hard time trying to give the second race 100%. About halfway through the 2nd race, I just started considering it a practice race (which isn't a bad thing).
My recommendation would be to pick one of these races as your "A" race for the day. This is the race you are trying to win. Then choose the other as either a warm up, cool down, and/or practice some different things. (some of this has been mentioned already) Placing won't matter in this race.
I'm thinking you should do both with the first as your warm up (hang back a little,watch and observe). Then stomp the other juniors in your second race!