BGP and aggregation

I have transit in 2 cities. I have a circuit connecting the 2 cities as well. So far I've been using non-contiguous IPs, so there's been no opportunity for aggregation. Having just received my /20 from ARIN, I'm trying to plan my network. Lets say I split the /20 into 2 /21's, one for each city. I'd like to announce the aggregate /20 instead of 2 /21's, as long as the circuit connecting the 2 cities is working. If the circuit goes down I want each city to announce the local /21. Is this possible? (using either a Cisco router or Zebra) Ralph Doncaster principal, IStop.com div. of Doncaster Consulting Inc.