Will adding a second SPF record mess up my DNS?

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.

An SPF record can include multiple domains (multiple mechanisms is the correct term), so more than one spf record will never be needed.

You can use this on-line SPF generator to get the appropiate TXT for SPF.