MS recommends a second dc for one major reason. If you only have one DC and it dies you lost EVERYTHING and must create AD from scratch.
That is no small task. Two DCs also share the load as far as name resolution [dns] and user authenications [connecting to shares and file/folder rights].
You should copy the dhcp folder info to the other server but don't do anything with it unless dc1 dies. Then you can authorize and walla dhcp is working again.
You can seize the fsmo roles if dc1 dies to dc2.
Lot's of reasons for two DCs. You just have to redefine what you consider "backup". If you want complete fault tolerant failover you need to cluster the two servers but that gets expensive and there is a learning curve.
You should seriously consider engaging DFS/FRS so you have a replica of your files and shares on the 2nd dc. Might come in handy someday...
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