How do the different speed step reconcile when double-heading?

Doubleheaded locomotives do not necessarily have to match perfectly in speed. As long as the "faster" locomotive is not spinning its wheels, you can run them together OK. The faster loco will pull a little harder on the train than the other, that's all.

If the faster locomotive spins its wheels while running, then that particular locomotive consist should not be run together.

Thanks Rich.  Is there any advantage to re-setting the speed steps in one or the other or is it a difference in the systems used in Sunset Models vs Atlas?

Rich, although your response may be true for 1:1 trains, I believe you can overload and burn up an O-gauge locomotive's motor without any evidence of wheel spin.

my santa fe f-3s a & b unit were not playing nice together the A unit got very hot trying to pull the B unit. to trouble shoot Lionel had me run them spaced one foot apart and time how long it too for the A unit to leave the B unit in the dust.  it was not very long so Lionel had me send them back.  after sending them back all is well.

The neat thing about the ERR TMCC Cruise Control is they have a "tweak" function that allows you to tune the two locomotives to match speeds.  As long as the gear ratios are the same (preferably very similar or identical units), MU works great.  I normally tweak one click past matching speed where the rear locomotive very slowly catches up with the lead unit.

Your new 3rd rail is using an ERR TMCC set up and the Atlas might be using EOB which could cause an issue.  I have not been successful in double heading anything other than either proto 2 engines or Legacy/TMCC (only in TMCC mode if both were not Legacy). To run mixed engines such as Proto 2 and Legacy or TMCC I need to use dual controllers to start them up properly. Using the Legacy or DCS controllers, identify the speed display on both controllers that match their pace and then you can run them at that speed regardless of the control system by dialing up those displays whenever you want to run them together.  John I have not tried the tweak function with ERRs kit.  What commands are used?  I could use that with my SMR Whiton and Tahoe engines to get the Tahoe to more closely match the Proto 2 Wooton.

Look on page 6 of the ERR Cruise Commander manual or page 17 of the ERR Cruise Commander M manual.  That's where the nudge feature is described.

Thanks for the information.  I will try to "tweak" my SMR Tahoe to more closely match my Proto 2 SMR Whiton.

Note that dissimilar locomotives with different gear ratios may not work all that well, the ERR cruise is depending on back EMF from the motor to sense speed.