Rather than using a separate guide scope, I steal a little light from my main scope through an OAG.
ZWO Off-Axis Guider
An Off-Axis Guider is a little sneaky thief that comes in and steals some of the light going through your main telescope. It sends this light to a second camera (called a guide camera) that then analyzes the scene. Using software such as PHD2, the computer watches to see if the stars drift. If they do, it sends commands to the mount to tell it to compensate to keep the stars in the same position. This means that the mount is being guided by the guide-cam by light coming through the OAG and it keeps locked on to the target much more precisely than without a guide camera. I use a ZWO Off-Axis Guider which is all set for a ZWO ASI120mm Mini or similar guide camera.
Now, you can use a second, smaller telescope to do guiding by mounting it next to your primary scope. This works well. But there are issues with this. Namely, flexure caused by the two telescopes being mounted independently, and also off-axis error where the guide scope may not be seeing the same thing as the main scope, and thus the guiding is just a little bit off. An OAG fixes this by making sure that they are all looking at the same target.
An OAG is not the best solution for every scenario. It's only stealing a small part of the light, so if there's no bright stars in that part of the main telescope's field of view, then it won't have anything to guide on. It also requires a more complicated (and therefore, more expensive)) prism as opposed to a less complicated (and therefore, cheaper) guide scope. But it is more precise, it has the same field of view, and it solve the problem of flexure without any complicated engineering. Plus, once you've got the OAG attached and focused, you don't really have to adjust it again. With a guide-scope, you have to make sure focus is solid every time. Unfortunately, it also means that if you want to take the OAG off, you have to redo your entire imaging train, so most people just put it on and forget about it.