SPF include vs redirect
Solution 1:
Pretty much what it says on the tin.
In the first case, domain 2's SPF record is included in the SPF record for domain1, but can still be modified eg by adding another A host that isn't permitted for domain2.com:
"v=spf1 include:domain2.com a:othermailhost.domain1.com -all"
In the second case, domain2's SPF record is used as the complete SPF record for domain1, and no further modifications are possible.
"v=spf1 redirect=domain2.com"