For this, I will assume that you're a) a consumer, b) you don't have any provider independent IP addresses ranges or an assigned ASN, and c) you're using linux on your router or are at least willing to.
With multiple connections, you would be able to provide link redundancy, static routing over preferred egress routes, and the ability to load-balance connections over both connections but you wouldn't be able to capitalize on the sum of bandwidth from both internet connections over one session.
I did this once at home when I had a Cable internet connection as well as a fibre connection. Cable was cheap and plentiful, but the fibre was oh so much faster and more reliable. In my case, I decided to put all VoIP traffic over fibre and wholesale data transactions over cable.
I recall referencing this site to aid in my configuration. I hope it helps!