Safest bet is to place BT's in mono, especially if it's the same source material on both sides. Stereo is only perceived by the listener if they're in the sweet spot between L-R. Stereo is great for a fixed listening environment but even then, the mix engineer is only trying to place instruments in the proper place within the soundstage image between L-R. Very rarely is that hard -panned L-R. That's really all it is. It's kind of pointless for outdoor venues, festivals, etc., where venue operators want as many people crammed in to the space - which usually includes directly in front of either L or R columns and even outside of them. Those folks may perceive stereo as "I can't hear the musician panned to the other side" - a bummer for those who want to hear something akin to the songs they've purchased or streamed and expect to hear something similar when they see you live. Mono will only require one input at FOH, be easier to mix for FOH engineer and will help avoid dreaded phase issues.
If it's any help, many of the best sounding recordings ever made were done in mono and they stand up favorably to their stereo re-mixed counterparts that were done later. There's absolutely nothing wrong with a tight mono mix. Besides, because you have two FOH speakers, you can always add stereo effects such as rotary, trem, auto-pan, short delays to voices and instruments to create a very convincing stereo image ala Chet Atkins. With that said, there are some sources, keyboards, or a Gretsch 6120 with the stereo pickups that sent the top and bottom strings to different outputs that are intended to have both L-R outputs and should be input with two channels - but, that doesn't necessarily mean they absolutely should, or have to be, panned hard L-R.
I agree that stereo effects can sound amazing and be very inspiring when you hear the sound coming back to you on stage. Standing between two guitar amps running wet / dry or dual delays is pretty cool. But, in the audience, especially in larger listening areas, it can be a completely different animal.
Just my two cents. There are no rules.
Good luck!