Can a home theater surround system be easily switched to just front left/front right two channel?
In the case of my Marantz (and Denon would be the same), there's no switching involved: Multichannel soundtracks will play as multichannel, while 2-channel will utilize just subwoofer and L+R speakers unless I switch to Dolby Surround, which as noted above, can do a pretty good job of simulating surround audio.
When shopping for an AVR or AV processor, do take some time to learn about the room-correction software used, as some are better than others in allowing you to create a frequency response which is equally appropriate for music as well as movies. As of late 2021, popular choices for enthusiasts include Audyssey's XT32, as well as Dirac and Trinnov products.
Speaker-wise, when playing movies, the Center channel does much of the heavy lifting, while L+R do surprisingly little. Whereas with 2-channel mixes, L+R do most of the work.