I want to run 802.11A for VoIP and 802.11G for data WLAN.
1. do I need a pair of antennas for 2.4g and another pair for 5g HZ? four of them?
2. does AP support dual-bands at the same time?
1) Yes. You need at least one for each band. Two is better for most implementations, for signal diversity.
2) Yes. There are two independent radios in the case, they operate with complete independence.