Will adding a second SPF record mess up my DNS, or will it be like adding an extra nameserver?
(i.e. it only helps, not hurts)
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From RFC 4408:
3.1.2. Multiple DNS Records A domain name MUST NOT have multiple records that would cause an authorization check to select more than one record. See Section 4.5 for the selection rules.
I'm not entirely sure what you want to achieve by adding a second record, but if it is something like adding extra hosts/networks as valid/invalid senders, you can probably do everything you want to with just the one record - just add whatever you wanted to the end of the line.
You can use this on-line SPF generator to get the appropiate TXT for SPF.