Is it possible to combine two iPhones with tethering to increase bandwidth or fault tolerance?
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.