To merge the SPF values, simply include all the parts into one single record, without repeating any mechanisms. Here's how to merge the following example SPF records for Outlook and MailerLite:
v=spf1 a include:spf.protection.outlook.com -all
v=spf1 a mx include:_spf.mlsend.com ~all
1. The SPF record always begins with the version prefix:
v=spf1
2. Both records contain an "a" mechanism and the MailerLite SPF record contains a "mx" mechanism, so include both in the new record:
v=spf1 a mx
3. Then add both "include" mechanisms into the record, resulting in:
v=spf1 a mx include:spf.protection.outlook.com include:_spf.mlsend.com
4. Lastly, we need to define how emails should be treated if they don't comply with the set rules. We do this by using the all mechanism. The last part has to be either ?all, -all or ~all. It’s not possible to have more than one, so it is ultimately your choice which declaration of all your domains will use.
The merged SPF record would look like this:
v=spf1 a mx include:spf.protection.outlook.com include:_spf.mlsend.com ~all