I'm not 100% sure how you're thinking of wiring this up, but I recommend being very careful. I wouldn't just connect two completely different batteries in parallel (and obviously not in series). Be sure you know what's involved and double-check the battery specs. Maybe it'd be better to switch back and forth between them.
You might not need to do any of this if your goal is 50 miles. Are you planning on doing 50 miles often or rarely? If it's only occasionally, you might be ok with just the Pedego 48v/15ah battery. With moderate pedaling, I can easily get 50 miles, including through some decent hills. I'm about 175 lbs; if you're a lot heavier or have a lot of hills, your mileage will likely be less, or you might have to pedal harder. Blue Monkey Bicycles, a store in Utah, did a range test and got 55 miles on the highest level of pedal assist. See their video here...
By the way, you said the 48v/15ah battery is a $400 upgrade charge, but unless someone is offering you a deal on the 48/10, the 48/15 is only $300 more, and definitely worth it.
Oh, and EM3EV is a very reputable company, so if they say they're shipping legally, it's likely true. I believe the owner, Paul, is a Brit ex-pat, so his communication in English is great. I haven't bought from them, but I've emailed once or twice. Come to think of it, that was a few years ago, so I don't know whether he still answers their emails, but I know their rep is still good.