If I am Silver elite, does it make sense to sign up for United cc and get 30,000 miles rather than signing up for CO cc (since both chase, I assume u can't open both within 6 months?). Or should I just stick to accumulating as many miles as possible on CO. I fly mostly domestic (most frequently NYC-DTW and NYC-LAX), though usually take 1 trip to Israel every year flying CO. thanks.