Multiple SPF records for multiple domains

We have recently started using Office 365 for our email, which requires us to add a DNS TXT record with the value v=spf1 include:spf.protection.outlook.com -all. We already have an SPF record with the value v=spf1 include:spf.mandrillapp.com ?all for Mandrill, an SMTP sender we also use.

My concern is this might cause some conflict. Is my concern valid or will be be fine with these two SPF records?

You can only have one SPF record (https://www.rfc-editor.org/rfc/rfc4408#section-3.1.2).
You need to combine them like so:

v=spf1 include:spf.mandrillapp.com include:spf.protection.outlook.com -all

or

v=spf1 include:spf.mandrillapp.com include:spf.protection.outlook.com ?all