I get this every now and then and we know both or either many not turn up.So double book and cross my fingers...and juggle!
As neither may ever be genuine....I'd take always take the first booking, Mr 1 to come and do him quick, and accept the second one's booking.
Direct Mr 2 sweetly to grab a coffee, by a text or quick call once he's arrived. If I'm with Mr 1 and need to text I say I'm need a pee break, glass of water or need to wash my hands.
Tell Mr1 I'm having a shower, mislaid my front door key (anything mundane and domsestic), so can he wait 10 mins, any old flannel.