Make sure you understand this before you pay money for Speedify, which I regrettably did: This is first and foremost a VPN service. It's not a bad one, you can pick from loads of locations, and the interface is actually not bad. But it's a VPN service, and one that requires a subscription. Don't think you'll pay $49.99 and are done with that.
Worse: in addition, for me at least, the promise that the apps's name implies is not being kept: this doesn't speedify anything. At all. If anything, it decreases the speed of my internet connection. And before you start: yes, I do have two separate internet lines, one cable, one DSL. Both are connected to my Mac via Thunderbolt adapters. With Speedify running, I get around half the throughput of what my cable line alone gives me without Speedify running. Only in the upload speeds I can see that the two lines are actually linked. But this isn't worth it for the steep decline in download speeds.
In short: if you're looking for a VPN service, you'd be better off with VPN Unlimited or something like that. If you want to combine two internet lines, be prepared that you might not see the results you were hoping for. You're much more likely to find the answer with a hardware solution: a router that offers two WAN ports, like the Draytek v2925.