I think it would put each of them on their own (forcefield equivalent of a) treadmill, and when they stopped and looked back at each other they'd actually be seeing holodeck versions of each other "far away", blocking their view of the actual much closer person.
Early TNG episodes showed that they would reach the edge of the Holodeck room, and essentially be "stopped" by a forcefield, letting the participants know they were up against the physical edge of the simulation.
I think they never got the scale of the holodeck right for TV in TNG. Different "decks" seemed to vary in size. Realistically, this should not be a 30' x 30' space. You need a gymnasium or warehouse sized room that is 10,000 square feet and at least 3 stories tall, to give participants room to move around.
ALSO: crew members had intimate "romantic interludes" on the holodeck, with holographic people. Can't see how, even in the ST Universe, you could make love to a hologram and have it remotely seem real at all. Unless the holodeck uses the same technology as the Voyager EMH, but then again???!!!
So my question is: are some of the NPC "people" on the holodeck actually robots/droids of some kind, computer controlled? Again, not advanced androids like DATA, but physically materialized synthetic beings (think how the food replicators work) with a robotic endoskeleton and artificial skin, that can interact with crew members in a more convincing way? An advanced love doll, if you will, that is PHYSICALLY PRESENT in the holodeck with the crew member, generated from protomatter (the same way the food replicators work) and constituted/materialized by the transporter, to be a real, physical "body" in the space, not just force fields and light?
First episode of TNG/Encounter at Farpoint. What strikes me here is that Riker has apparently NEVER seen a holodeck?!
DATA explains it to him, how much of it "is real" using transporter technology to create "real" rocks, plants, and trees in the space. Also, they get up against the wall.
So.... if two people tried to run in opposite directions in a holodeck, I feel that you would each reach the edge of the room, and not be able to run any further, rather than have the room put you in a "forcefield treadmill" to make you think you were running forward.