HA with non dedicated heartbeat interface / 80D

Hi, I am implementing a small network for public access wifi at a conference centre, 10 fortiap, 2 fortigate in HA and 2 WAN broadband connections. The solution needs 2 WAN ports and 1 internal port. The fortigate 80D is the right size for throughput inc web filtering and capwap to the access points, but I have a concern as the 80D only has 4 interfaces I will not be able to have 2 x dedicated heartbeat. Can I configure 1 HB across dedicated xover and one shared with local LAN, or other solution that might work in this environment. Thanks in advance James