When buying 2 bikes, where you are often riding together, getting bikes that share components can be very useful (and I speak from experience).
If you can swap batteries, when one of you wants to go on an extra long ride, you can take both batteries.
Also, if one person is struggling or gets hurt and needs to run on a higher assist, having that second one to swap might make a big difference to getting home safely.
Also, some retailers give discounts if you buy two at the same time.
For example,
DOST bikes (new Canadian online company) gives $250 discount when buying 2.
So 2 of their bikes would only be slightly over your budget in USD. ($5348 for two bikes)
I just used DOST as an example (as they're sold out until the Oct/Nov shipments are in) - I am sure many other companies (LBS included) would have similar incentives.