If it's MFM, then both men are engaged at the same time. Very much unlike a train where one dude gets off while the other one waits his turn and then goes.
As far as upcharge goes, it really depends on the lady, but no lady is only going to charge their one hour rate two entertain two men at the same time for that one hour. They will at least (for me) will be paying for the full time individually, which is not an upsell at all, but just a combination of two separate dates happening at the same time.
I also wouldn't blame ladies for charging more as this could definitely fall into PSE especially if there is dp, and PSE is always more expensive than gfe.
As for me, I have never done MFM but it is definitely a fantasy of mine, dp included
I think is would be amazing to be entwined between two warm bodies, 4 sets of hand, 2 mouths, and 2 cocks at my disposal ready to please me and explore my every nook and cranny.