Hi @NJacobe
Your premise is correct - you would be able to balance and utilise the bandwidth across both lines.
However as @jdsilva pointed out, you will be prevented by the throughput available on the MX100 of approx 650 - 750 Mbps depending on your configuration.
Its worth noting you have a few options with the load balancing, you can use both lines together or you can use one as a failover, which may be more suitable given the hardware you have.
To ensure balancing and failover work correctly its important to set the line speeds in SD-WAN & Traffic Shaping. Below is a shot from an MX100 configured with two lines.
Some good pointers here in the Load Balancing & Traffic Shaping documentation
Cheers
Nick