Obviously you can… Fulvio explains it in the same how-to when he says that you can create a layer 2 VPN tunnel and bond these together.
That’s usually how all aggregation is done regardless of the connection used.
As far as static IPs is concerned, I recommend registering each Zeroshell device with a dns server online and so getting a hostname.
It works as I’ve tested it myself.
And just my two cents worth:
If you’re opening this topic it means you’ve not done enough research as all info is available in these forums and Google…