Sharing resources between Asterisk servers

I have an Asterisk server which places outbound calls on a 4 port Digium card. The customer wants to add an Asterisk server with additional Digium card(s) to increase outbound capacity.

I have read about peering but have not seen anything regarding groups. On the currently running server I can specify a group in the channel field (Zap/g1/8005551212, for example) and would like to do something similar with the Zap resources on the second server. It appears that I would specify channels on the second server something like this: Iax2/outbound/8005551212.

How would I load balance the outbound calls between the 2 servers if I must specify the channels (and groups) in this way? Is it possible to put Zap channels on server A and Zap channels on server B in the same group?