Two voltage regulators on same circuit
Of course you can power multiple regulators from one battery, however, there are a few things to be aware of.


Battery voltage will drop as it is depleted. You need make sure your regulators won't drop out when this happens.
Lithium ion batteries can be damaged by being excessively drained. You need some kind of battery monitor and undervoltage lockout. 
You will be wasting power in the regulators. I know the Pi draw significant current. A SOT-23 regulator is probably not the right part to power it. A TO-220 would almost certainly be fine. Do the math.
At a minimum, fuse the battery. It might not be a bad idea to fuse each regulator individually. Drawing excessive current from a lithium ion battery is another great way to damage it. 
If you're thinking about charging, study lithium ion charging carefully before even thinking about a circuit. If you doubt me there, Google "hoverboard fires." That's a topic for another question.


Overall, I would recommend a couple of buck-boost regulators to maximize battery life.