I'm guessing you mean for music sources as phone calls, videos (most), TV streams, podcasts, ebooks, etc don't have enough true stereo separation to be a problem.
I have several Bluetooth mono headsets that support A2DP and they combine the streams automatically. Both Jabra and Plantronics make them, most are around $50 these days.
I like the Jabra 8010 best, it's older and kind of big, but it has a huge battery and very loud volume. I like using just one when playing games, or videos, podcasts, etc in public. Let's me "keep one ear open" as it were.
Hope this helps.