Using an LACP trunk on 2 WAN interfaces will only increase your bandwidth if you are using one router, AND the link between FGT and router is saturated. I highly doubt this.
If you want to combine 2 ISP links to have their individual BW added up then you can either
• use one router capable of link bonding (viprinet.com for instance)
• use load balancing in FortiOS
But, both ways are different and have differing benefits.
Load balancing is done on the session level. Only complete sessions (transfers) will be using one of the 2 WAN links, alternatingly. With 1000s of sessions of comparable BW allocation this may well work for you.
Link bonding on the other hand will combine data streams on the bit level. This is far beyond the capabilities of a FGT. Link bonding routers can combine media of all sorts, like DSL, 3G/4G, cable/ethernet and so on. But they come at a price whereas FortiOS features are paid for as a whole.
"Kernel panic: Aiee, killing interrupt handler!"