"on IOS, it seems only 1 destination is allowed for command "ip helper-address"
Not sure where you get this impression from. You can configure multiple ip helper-addresses under a L3 interface. Not sure of the actual limit but i have used up to 4 in a transition period. Be aware that if you have multiple ip helpers the router/L3 switch will send a DHCP request on behalf of the client to all those listed.
There is one caveat, which hopefully you won't run into. Certain devices like the ASA can get cranky if you have multiple DHCP servers load balancing. If they don't get a reply from the server their relay agent asked by unicast and get it from the peer instead, they'll drop it, even if you listed both the DHCP peers in the relay line.
We had to move our dial-in zones to only be served by a single server.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...