The usual suspects as room acoustics, matching the size of the room to the volume in the room, not putting cabs parallel to the walls or even floor, getting them off the ground for less reflections, comb filtering, checking the reflections inside the cab (with damping the inside of the cab if needed) and generally putting the cab where the reflections combine well with the direct sound aside... there are time alignement plugins that help. Two sources can be time aligned in the DAW without too much pain, some phase alignement tools can be handy as well. If you got neither, plug/unplug the guitar, that makes a short and peak-y noise with which you can align them by looking at the waveform on the screen.. if the resolution of your waveform by chance is making it possible. Still, if your phase/time difference is just inbetween the measure of the sample frequency, you're SOL. It's like you can, with software, just nudge your data in solid increments, not inbetween them. Here's where neat stuff like the Little Labs
comes in. As well as the old method of using ears to properly match phase on sources, which, with differing distance, isn't always 100% possible. I, for my part, have come to the conclusion that putting more than one mic on an identical source (which it really isn't, because no two spots in front of a cab are
sounding identical) is not worth the trouble. I rather use one mic on one cab and split the guitar to another amp/setting/cab to get a more pronounced signal that provides what the other might be missing. But hey, it's not like my shtick works for everyone, for others it might be completely off, according to how they hear stuff. Take my words with a grain of salt, will you, please?
The other route would go down the "make the source less loud" so you could place the ribbon and the other mic without having the ribbon work on the edge of it's comfort zone.