When thinking about how to ground things, I find it helpful to think about where currents flow. Remember, ground isn't magic. It's just a copper conductor like every other copper conductor in your circuit.
Also, remember that circuit means you can start at a point, follow a path, and end up back at the start without retracing your steps. Current only flows in circuits. There are a fixed number of electrons and other charge carriers in your electrical device, and none enter or leave. You can only push them around in circles. It's easy to forget this when we use ground and power rail symbols to simplify our schematics, but if you draw them out as they really are, it's clear that current always flows in a circuit:
simulate this circuit – Schematic created using CircuitLab
current flows in a circuit, through the battery, even when we don't draw it
If you have a large current coming from the power supply, then you also have a large current going back to the power supply through ground. The goal of a good grounding scheme is to anticipate where these currents will be, and keep them away from places that will be problematic. Wherever a large current is flowing, there will also be a change in voltage, because your ground will have some resistance and some inductance too. Since your signals are usually defined by a difference in voltage from ground, if you change "ground", you also change the signal. That's problematic, so keep large currents away from your signals.
If you connect your signal and power grounds in only one place, then you know all the return current for your power devices must go through that one point. For every unit of charge that enters the power devices, another unit of charge must leave it at this one point where the grounds connect. This makes it very easy to understand where the currents will flow: you know they will not be flowing through your devices that measure the sensitive signals.
If you connect the grounds in two places, it becomes much harder to say for sure what will happen. Where the currents flow now depends on the relative inductance and resistance of the multiple paths and so on. Now maybe you are sharing a current return path with something else, and as that something else switches on and off, your ground moves up and down. Of course, having no other reference, you can't tell the difference between your ground bouncing up and down and all your signals bouncing up and down, and the result is noise in your signals.
So, when deciding how to connect things, here's what you do. Ideally, all the power supply connections, both of them, aren't shared with any other device. Everything gets one wire, not shared with anything else, running directly to the + side of the power supply, and another one to the - side. You can then be sure there are no currents, and thus no changes in voltage, except the ones you cause. When this isn't feasible, ask yourself: if I share this path with something else, what currents will it cause? Will that be a problem?