Is it possible to combine two iPhones with tethering to increase bandwidth or fault tolerance? I'm using a MacBook as a "server" and want to keep it always connected to the internet, over cellular. 

I have two mobile phones (one GSM, one CDMA) and would like to connect to each of them to achieve fault tolerance, or ideally, increased bandwidth.

Is there any way for me to


Simultaneously connect to two iPhones 
Use the joined connection for fault tolerance, or increased bandwidth 


I have the option to use Wifi, USB-C, or bluetooth to combine these phones, in any combination
Technically speaking, you can connect two iPhones simultaneously; it's called multi-homing (this is good for fault tolerance).  The pf firewall has this capability and is built into macOS, but writing the rules file is outside the scope of this question.

However, you can't combine the bandwidth to get a larger "pipe."  You'll still have two smaller pipes.   In order to combine bandwidth, you will need to aggregate the pipes or bond them which is something that the ISP/carrier will need to do for you - different carriers (GSM and CDMA) are very unlikely to do this.