I was wondering the same thing. I mainly use Croma blades and have two razors. I shave every other day and started using two razors a year or so ago. It works very well. With my Merkur 1904 open comb, I do a WTG and XTG pass which works very well for beard reduction.
Then with my gentle EJ 89, I do another XTG pass and then finally an ATG. If I were to do the last two passes with the Merkur, it would get bloody. If I did the first two passes with the EJ 89, it would not reduce the beard as much.
This combo works very well although I will likely get a Slant soon to mix things up.
I did my first pass (WTG) with a Trac II cartridge razor. And I did a second (ATG) pass with a DE razor (Gillette G2000).
Since then I've learned to use a DE razor properly, and I don't need the Trac II anymore to get a BBS shave. I still use the Trac II as a travel razor though.
I mainly use 2, sometimes 3. I use my R41 for the area under the sideburns, a slant for the first wtg pass, and a Future for the next xtg pass. I finish with the slant again on my neck; mainly atg. I find the Future (on 5.5) just can't cut through the thick stubble below the nose and chin. PS - I use Feather blades, except in the R41, where I use Personna Blue. The R41 is too much with a feather.
If i decide to shave with my DE Muhle R89 Grande (awesome awesome razor!!), instead of my G&F Straight Razor on a given day, i'll either use the straight or my Parker shavette just to get a straight line on my side burns. Not always though, i've now tweaked my DE skills so i can get straight lines with my DE, but i find it easier to shape facial hair with a straight. But in terms of the actual shave, i usually use 1 razor for the shave elements.
The only time I shave with two razors is when I want to do a head-to-head comparison of two razors (or blades, in which I'll use two identical razors and different blades).
When learning I used my cartridge razor to clean up. Now that I'm comfortable with DE, I often use it to do my lip and chin area, as I learn to use a shavette, and eventually a straight.
Yes. I use a Schick Dial single edge to trim my mustache and then use my DE to shave the rest of my face. I tend to wear a pencil mustache, so the single edge helps me keep it neat.
Right now I am teaching myself to shave with a straight so I have been using 2 razors per shave. I'm using a Magic shavette and my Slim. I'm starting to get to the point that I don't need the Slim anymore, though.
Each of my razors were different. And no one could do a job from beginning to the end. So my 1st pass was not much pleasurable with R41, I have not reached required closeness with EJ 89, I enjoyed smoothness of Gillette 2000 and dreamed about one single razor to get all the best of these three. Then I found Schavette. Potentially it could do that, but I feel there is a learning curve that I did not passed yet. And finally I found devette - DE razor without a base plate, i.e. without a comb or safe quard. I use EJ89 this way. And need no more razors, this way I get exactly what I want - I feel the blade, not the razor on my skin. It is not more dangerous than R41, I have never had any cuts this way and no more irritation. And I can think no more razor...