I have been in bands that have done all the methods mentioned above, I always liked the Small amp leaned back on the front of the stage the best.
we had two guitar players in a band a while back and tried the side fill amp, It worked ok except the volume was a lot higher as the guitar players, where hearing the other guys amp better than their own, so with two, you may need to raise them up closer to ear level or place the amps on the opposite end of the stage from the guitar player it belongs to. So they can hear it.
I was doing sound at a venue in KC and we had some baffles to place in front of the amps, they where made out of some kind of carpet and plywood. I do not think plexglas will do as well as these do.
I found if I used these and placed them out in front of the amp, a couple of really nice things happened.
One, it killed the blast zone right out in front.
Two the guitar players liked it, with this baffle placed 12 to 24 inches in front of the amp, they could hear it, it did not effect the tone much and in some cases they could turn up a bit.
Three I also found that I could move the mic away from the amp and in the case of a combo with two speakers I could sort of capture both speakers and the mic was kind of isolated from other things and did not seem to pick up much stage wash from drums or other things,
I do not like amps leaned back at the rear of the stage as it seems, they are always aimed right into a vocal mic.
Most of these are combo amps.