You can do direct-wired sync, and it is supported, but it will cause problems. Not may, will. The TAC had already recommended against direct-wired sync for years when I started in 2006.
One of the things a cluster member monitors to determine its health is its interface status. With direct-wired sync, if one member dies or is rebooted, the remaining member sees its interface go down. This is a failure, and the member can't tell if its interface died, or if something outside it failed, so it causes the member to evaluate whether it is the healthiest member left in the cluster. This is a pretty conservative process, as you really don't want a member whose NIC failed thinking it is healthiest and claiming the VIPs while the other member is also claiming the VIPs.
Among other things, a member checking to see if it is healthiest will ping addresses on all of its monitored interfaces (and on the highest and lowest VLANs of monitored interfaces with subinterfaces). If it gets a response, the interface is marked as good. If it doesn't get a response, the member interprets this as further evidence that it is the one with the problem and therefore should not take over the VIPs.
This can easily lead to situations where you reboot one member to install an update, and the other member thinks it has failed and goes down.
The best way I've seen to run sync is two interfaces in a non-LACP bond (round robin works) going to two totally separate switches. If one switch dies? No problem: the bond just handles it. If one member dies? No problem: the other member has link on all of its interfaces. If the link between one of the switches and one of the members dies? No problem: the members realize that sync is broken, but the interfaces are still up, so no failover is triggered; you just lose redundancy.