You can only accomplish that with the use of let say bittorrent or a download manager. The problem is not the fortigate, but how TCP/IP works.
lets say a user downloads a file from a FTP site,
the fortigate uses WAN1 for its connection, and wan1 only have 6 mb, if you were to have a download manager then it could connect to the ftp site on wan2, and both connections could work together and take every other byte from the file, then you would receive 12 MB.
however you can still use both wan connections, just set them up as usual, and be shure to give both static routes the same metric, dont forget to create firewall rules too =)
FortiGate 200A/B, 224B, 110C, 100A/D, 80C/CM/Voice, 60B/C/CX/D, 50B, 40C, 30B
FortiAnalyzer 100B, 100C
FortiAP 220B/221B, 11C