How can I combine internet from two sources on my macbook and get increased speed?
What you're referring to is called Internet Bonding.

You can't simply turn on two different Internet connections and have everything work - you need an intermediary whether it be a dedicated router or 3rd party software to properly manage the packets as they split between the two connections and then have them reassemble back on the other end.  

It's very similar to Link Aggregation but instead of being internal where your host (computer) and switch must support it, you must have an ISP or a 3rd party service that will do this for you.

Here's a few companies that do Internet bonding:


Speedify 
XRoads Networks
ZifiLink (3G, 4G, LTE bonding)


Normally, this is done at the Enterprise level where a router can have multiple WAN connections (I did this with 3Com/HP and Cisco gear) or in remote location where broadband was more expensive than multiple cellular data networks.  

As for software, there's


OpenMCTC Router
pfSense Multiple WAN connections


In these instances, you're acting as the bonding service provider.  Given that it's available on pfSense which is the same firewall that macOS uses (pf) it's conceivable that you can bond multiple Internet connections using the built in firewall.