No. They are somewhat related, but they are different services.
IFC is the cable channel. Most people watch it on their cable set-top box or they log in to ifc.com using their cable/satellite credentials. You need to have a TV package that includes the IFC channel.
IFC Films Unlimited is a stand-alone movie streaming service. It only contains films, not any TV series. IFCFU was launched fairly recently, long after the IFC cable channel was established.
Can I sign into both if I only have an account for one of them?
No. They are different services with similar names.
However, if you really like the content from both, there is a great option for you: get AMC+. AMC+ includes the IFC livestream (identical to what cable viewers see) as well as some of its movies and shows on-demand. Plus it includes all of the content in IFCFU. To learn more, go to www.amcplus.com.
Do they have any of the same content?
Sometimes yes. It's not especially common, but some of the films on IFCFU may also appear on IFC.