Dual Cellular Modems

I bought an PCEngines APU3 so I could experiment with dual cellular modem connections.

What are some of the ways I could utilize dual active PPP connections?

LAGG is what I found. Any other ways?

In failover mode it says:
"make sure that the underlying physical interfaces are up, then create the lagg(4) interface."
This could be problematic as the modems USB interface comes up late.

I am not sure if I can even do LAGG failover with dual PPP or MPD5 tunnel devices.

I am using ATT network now via Straighttalk tablet SIM and I am going to try Verizon next.
The modem I am using is the Sierra MC7354 and MC7355. There are separate firmwares for the different mobile carriers.
I want to use GSM and CDMA for a mobile router.
What I did was make 2 dialing entries in my ppp.conf. One for each device and I use -ddial to connect each mobile boadband device to their networks. Now comes the fun part. LAGG with DHCP.
I can only imagine that creating an rc.conf.local with any LATE entries you might want to start, could be added there. Not unlike mount_late.
You could use it to fire up a script that opens with a sleep(1) statement, or something. But honestly; I think you're making the best choice creating the ppp.conf entry. I'm imagining that this is intended in addition to utilizing a couple of entries in ttys(5)?
That's the way I'd do it, anyway. I have to add an entry everytime I "root" a new phone. <evil grin>