I am looking to setup a TXT spf record that has 2 included domains... individually:
v=spf1 include:_spf.google.com ~all
v=spf1 include:otherdomain.com ~all
What is the proper way of combining them into a single item?
There's no restriction against including multiple names in a single entry; Hotmail, for instance, takes this to extremes. Note that multiple includes, or nested ones, need to stay under the limit of 10 total DNS lookups for the whole SPF check.
It look like you can merge both together into one entry. Lenovo.com appears to be an example of this.