Maybe have the two services run separately and then keep them in sync with some syncing program?
To what end? I am as much a fan of redundancy as the next guy, but that idea is simply overkill and ripe for issues. The only upside to cloud over USB drive is for site disaster. Fire, flood, what have you.
IMO, a smarter more effective thing is one cloud service, take your pick, your system HD and an external USB drive. Sync the 2 HD's and you have a quick solution for a HD failure, plus an off site backup for a home office fire/flood etc.
Combining 2 cloud services is just not smart, IMO. The cloud service has multiple redundancies built in, so we don't have to.